Granite balls-up

Dr Livingstone was meeting some mates at the O2 Arena (formerly known as the Millennium Dome).
Leading up to the entrance on Peninsula Square there are paved lines that run parallel to thePrime meridian. They are about 18 meters apart, that would equal about one tenth of a second of longitude.
Here and there some words on granite slabs interrupt the lines. All text relates to Greenwich, time, longitude etc.

Following text on tile struck me:
"John Harrison - 1764 Invents the H4 marine chronometer"

Call me a pedant any day, but that date stinks.

As mentioned in a previous post, I have been reading the little Dava Sobel paperback Longitude on the life of John Harrison. I'd picked up a copy in a second hand bookstore.
And a very interesting read it turned out to be. I recommend it to everyone with an interest in, be it, history, navigation, astronomy or the science of clocks.
So, why is 1764 wrong?

From memory, this has several reasons according to me:
-H4 was completed in 1759. If the date is anything to go by; there is a signature near the plate's perimeter that reads: 'John Harrison & Son 1759'
I'd say that's an open & shut case. I could only make an allowance for a later date based on the acknowledgement by the Board of Longitude.
-H4 was presented to the Board in the summer of 1760, but no where near the 1764 date.
So what about receiving actual recognition of the H4? Could we fiddle that date and push it a bit later?
Erm, no. Granted, H4 had two trial runs. The Board was not very keen on handing Harrison the 20,000£ and there was one member eager to promote his own lunar distance calculation and reap the reward.
So the first trial run was in 1762 and the second some two years later in 1764. And they only agreed in 1765 the watch complied with the Longitude Act. In 1773 Harrison would finally receive the full reward for solving the longitude problem.

So to hang the issue on semantics; in this case H4 was not 'invented' in 1764. It had been completed a good 5 years earlier, after two years of construction. And it was not an entirely new item. H4 bears a very close resemblance to a watch for personal use designed by Harrison and executed by Jeffreys in 1753.

So there. That was my little rant on the granite balls-up.

H4 can be seen in a display case in the National Maritime Museum in Greenwich.