Unknown Coin | Coin Weights

It is indeed a "coin weight", used to try to identify counterfeit foreign coins in circulation. But the symbols indicate the value (in British predecimal money) of the coin being tested. The L is a pound sign, £, while the other letter is a cursive S standing for "shillings".

In Britain in the 1700's (and presumably in British colonies too), a Portuguese gold 8 escudos coin (also known as a dobra) was reckoned to be worth £3/12/-, so I'd assume this is a coin weight for a dobra. If I'm right, your object should weigh about 26.8 grams.

These coin weights are now collectable in their own right, though of course they're not as valuable as the gold coins they once represented. This one went unsold on ebay for £22; perhaps a little pricey.

It is indeed a sovereign weight. I assume this is the one you've got.

In this case, the "5.2½" stands for "5 pennyweight 2½ grains", as you can see by the tiny "dwt" and "gr" above the numbers. Pennyweights were way before my time here in Australia, but I'm told by Wikipedia that they're equal to 1.555 grams, with a grain 1/24th of that. So, 5 dwt 2.5 gr should equal 7.936 grams... which is pretty close to the 7.988 grams book weight for a sovereign.

In the link I gave above this one is quoted at being "Withers 2251a" - presumably "Withers" is a catalogue for such weights. The dealer is selling that particular piece for £20.