[Granitehill]

I've got one of these - I think mine was manufactured in 1973. The instruction plate on the back has the legend "12B R.P.B 73/5". I've no idea what R.P.B stands for, I'm afraid.
Mine is identical to yours, except for the panel lettering which shows "P.O. M.M-R No.12B". I can only assume yours is a later revision, since yours shows "No. 12C". I think the manufacturer was SEI (Salford Electrical Instruments - later taken over by GEC).

Should you need to get inside the meter, there are two screws concealed under the instruction plate. This is glued in place, so you will have to carefully peel this up with a thin blade. Double sided tape will stick it on again.

Nicely made inside - all I had to do was apply a wee bit of contact cleaner/lubricant to the AC/DC toggle switch and the track of the ohms adjust pot. It gets used most days, and is my first go-to for quick non-critical measurements. Accuracy is OK, but AC measurements are a bit out - it reads about 10% high on the 250 VAC scale.