Glad you have reached this conclusion. I had been thinking of coming back to be a bit more assertive about serving tongs rather than surgical tool. As an aside, I take a passing interest in Roman flatware (only passing because most of what you see is of doubtful provenance and often fake). One of my pet hates is the way dealers will advertise a "Roman medical spoon" when they have a perfectly ordinary small spoon with no evidence about its original use.
But to return to your tongs, in your very clear photos the mark seems to me to be a definite S.T rather than, say, IS but you have the advantage of seeing it directly. I rather fear it may be one of the many cases where we fail to make an attribution but let's hope there is somebody who knows better.
A far as the 4:8 is concerned, if it represented a weight in some measurement system that converted to approaching 175 grams, it might help suggest a country of origin. However, I don't spot a candidate so I am left to assume, like you, that the tongs are probably British, perhaps provincial or even colonial. The 4:8 may therefore be something like an inventory number (e.g. a group of serving pieces were kept in drawer or tray number 4 and this was number 8 of those pieces) but that is just speculation.
I have to say that I think this is an interesting item and a good find, so congratulations Good to think that the exchanges between our experts on Russian silver can have this sort of spin-off benefit!