As you can see, it looks just like all the other antique dice I have, except in one regard: there's a faint red mark on the 6.
That faint red mark is a tax stamp. Dice were taxed in the UK from 1710 onwards at 5/- a pair, and heavily so (as in £1 a pair) in the Stamp Act of 1765 that applied to the colonies in America (repealed the following year as they kicked up a fuss about it). The tax in the UK went up to £1 a pair in 1804, remaining at this level until the tax was discontinued in 1862. Dice duty was widely evaded: in 1834 the treasury received £1,000 in dice duty but MPs did not believe the implication that only 1,000 pairs of dice had been sold that year.
In 1801, pairs of dice were sold in wrappers with tax stamps on them; the dice themselves were stamped with a crown. Prior to that, they had no wrapper but were stamped with a crown plus the initials of the monarch. In my die's case, there's a crown above the letters G.R. (for George Rex). The George in question is George III, so this die dates from the 18th Century. You can probably just about make it out from the scan, but I have a magnifying class so I can see it a bit better.
There were two G.R.s immediately before George III, but I don't think the die looks old enough to be from one of those. I can't find any examples of their duty stamps on the Internet, either, so it seems unlikely.
Hmm, I wonder if it still works?
(roll, roll)
Heh, I got a 6 then a 1, so yes, it does.