"Tower" pistols referred to any British military pistol assembled at the armory of the Tower of London, over the 1740-1840 period. Originally they were all flintlocks, then there were converted flintlocks mixed with production percussion pistols. There were different versions for the Navy (Sea Pattern), the cavalry (including long-barrelled and short-barrelled versions depending on the type of cavalry), and the infantry (Land Pattern).
Based on the assorted versions I've seen, it looks to me that it's a flintlock-to-percussion conversion, since the purpose-made percussion variants all seem to have sort of boxy stocks with squared-off grips at a sharper angle/curve instead of that more gentle curve, and don't have that big brass butt cap on either. You can see what appear to be plugged-in holes, where the superfluous flintlock parts were removed.
Now, normally you'd see a royal cypher under the crown on the lock, and if it was a conversion, it'd likely be G.R. for George II-IV. W.R. for William IV or V.R. for Victoria would be possible but by Victoria's reign they'd be purpose-made percussion pistols. Your example doesn't have one that I can see.