Most of the slugs have a number 5 stamped on them

the mark says AR for Albert Rodrian, one half of RaRo. If you ever see an R inside an octagon thats the mark for the other half, Alfred Ransmayer.

7.6 is the motion (distance between the baseline of lower- and uppercase characters) in millimeters.

The trident is the foundry mark of Gordon Webb & Co., a British type foundry. No catalogs for this company exist on the internet so can't tell you the name they used for it. https://i.imgur.com/RQpxK05.jpeg

The AR foundry mark stands for Albert Rodrian, one half of RaRo. (the other being Alfred Ransmayer, Ra type foundry mark is a R inside an octagon.4[2:39 AM]