5 Shilling Victorian stamp

5/- QV stamps are not uncommon. I have several in my collection. They were issued in the 1870's. I also have several 10/- and £1 QV stamps from the same period. 10/- and £1 stamps were issued under all the monarchs since QV, but no £5 ones, at least not pre-decimalisation.

The highest denomination QV stamp was £5, which, if inflated to today's values, would be fractionally under £600.

Some £5 stamps in collectors' hands (not mine!) were franked 'Hatton Garden', and AIUI were for consignments of diamonds or other high-value jewellery. The cost of the stamp included insurance, presumably against theft or other loss, and presumably not necessarily diamonds or jewellery, but other high-value items.

Images here, but I don't see a Hatton Garden franking.

although there is one franked 'Threadneedle Street', so was presumably from the Old Lady. Gold bullion perhaps?

Could be either. Bear in mind she appeared om stamps for the dominions and colonies; and on both postage stamps and "Revenue stamps" (for stamp duties). Oh, and some Revenue stamps could also be used for postage.

Hear endeth my knowledge of Victorian stamps: I once worked on stamp duties but I ain'tn't that old.

Yes, more likely Stamp Duty on a share certificate or something of that nature.