Here is another example of the Strathearn "Tropic with bubbles" weight.
This one has an oval facet which has been engraved with '56 Reconnaissance Corps', otherwise it is of a very similar size, weight and construction.
The 56th Reconnaissance Regiment was founded in January 1941. They would have had a 40th Year Anniversary in 1981. It is very possible this weight is to commemorate that event. This would tie in with the dates Kevin provided, allowing for sufficient lead time to make the weights for such a commission.
My wife thinks I'm crazy for buying this, but it was a good price and how could I pass this possibly datable monster by?!