They say “Every day's a school day.”

Well it certainly is when you're a stamp dealer!

I've learned a lot during my research for this email. It's one of the greatest pleasures of my job.

Another is the honour of handling valuable stamps any collector would love to own. And I did that this week as well.

Because today's featured item is incredibly rare.

It's in spectacular condition.

And what's more, I believe it's significantly undervalued.

In the right collection, I'd say it's a major showpiece – but without the major price tag you'd expect to pay for such quality.

Our story begins in Australia on Valentine's Day, 1966...

As the nation makes one of the biggest changes in its history.

This is Dollar Bill.

If you're Australian, and of a certain age, you'll remember him well.

Because in the mid-1960s he had a very important job - to explain decimalisation to the public.

Australia wanted to convert to the decimal system as early as 1901, but as part of the Empire it needed the support of the British government.

And unsurprisingly, they said “no chance, now clear off.”

It wasn't until 1959 that the Decimal Currency Committee was formed, and strongly recommended the change to help boost trade.

But this time the Australian government asserted itself. Shillings and pence were out.

They announced the decision in 1963, and held a public competition to suggest names for the new currency that would replace the Australian pound.

Notable entries included the Austral, Oz, Boomer, Roo, Emu, Koala, Digger, and Dinkum.

Thankfully common sense prevailed, and the government decided on the 'dollar' - although having a wallet full of Koalas would have been fun!

The conversion date was set for Monday, 14 February 1966.

Obviously this change required decimal postage stamps to match, and a set of new definitives was designed.

The lowest values of 1c., 2c., 3c. and 4c. featured a common portrait of the Queen, with higher values featuring birds, marine life and famous navigators.

The new decimal stamps actually went on sale week earlier, on February 7, but weren't valid for usage until February 14.

And then on September 9, 1967, to reflect the increase in postal rates, a new blue 5c stamp featuring the Queen's portrait was issued.

A perfect imperforate.

Records state 1,672,194,000 of the Queen Elizabeth II 5c blue stamps were produced by the Note Printing Branch.

And amongst that vast number, a small handful of sheets were printed with an imperforate error.

The error showed the first two columns imperforate, and the third column imperforate on the left.

Due to the high levels of quality control involved in modern stamp production, this was unusual in itself.

But almost all of these sheets were swiftly discovered and corrected in an equally unusual manner.

They were passed through the perforating machine a second time, which corrected the original error, but left the rest of the sheet with double perforations.

I've occasionally seen strips of these scarce 'double perforation' errors at auction. But far rarer are those which escaped.

Just two of those original imperforate sheets survived uncorrected.

These were broken into two horizontal strips of ten, each with two imperforate, and double strips of 20, each with four imperforate.

And that brings us to today's superb featured item.

The Australia 1966-73 5c deep blue error.

This block of six originates from one of those double strips.

It's been described as a “spectacular modern rarity” and I wouldn't hesitate to agree. It's a marvelous piece.

As you can see, it's an upper left corner block comprising two horizontal strips of three, each with the left pair error imperforate.

The condition is exceptional: unmounted mint with full original gum.

And from an aesthetic point of view, this is a very nice piece to look at.

The portrait of the Queen by Anthony Buckley is wonderful, the deep blue colour is striking, and it's in an attractive positional form.

Having seen it up close I know it will absolutely shine in your collection, either on an album page or in an exhibition frame.

Undervalued on the market - for now.

As I mentioned earlier, I've always personally felt that the rarest Australian stamps are undervalued on the market.

The country has a rich philatelic history, which offers so much depth and variety for you to study.

From the earliest colonial stamps of the 1850s, to the birth of the commonwealth in 1901;

The first iconic 'Kangaroo and Map' definitives; the commemoratives celebrating Australia's achievements, its unique flora and fauna and indigenous cultures;

And the gradual move from British rule to independence, depicted in the changing imagery over the generations as the nation found its own voice.

I've certainly enjoyed owning a few good Australian stamps over the years, and every one has taught me something new.

The block of six I'm offering you today is a great case in point.

The catalogue value is double what it was 20 years ago, which translates as 5% simple growth per annum.

Now that may not seem spectacular - but it shows it has comfortably gained and held value in a changing market, and through some very sticky financial times.

And I feel it offers superb value for money, when compared to similar items from other areas.

One that instantly springs to mind is this Cyprus 1934 1pi black and red-brown left marginal block of four, comprising two vertical pairs with the error imperforate in-between.

Both items are highly rare, with a strong appeal to Commonwealth collectors.

But the difference in the price illustrates why I feel major Australian rarities are currently significantly undervalued.

We sold this imperforate error block for £50,000.

Yet the very similar left-hand marginal block that I'm offering you today can be purchased for around one tenth of that price.

So if you're looking for highly rare, visually striking Commonwealth rarities to add to your collection, it's a piece you really should consider.

Own this magnificent Australian rarity today.

Stamp collecting has had a major boost in recent years.