Nothing I can really help you with that you don't already know. This is British Pattern 1903 for use by all arms equipped with the rifle other than Cavalry & Mounted Infantry. During WW1 this was standard kit throughout the Commonwealth for all arms with the rifle other than infantry, so the main users were artillery, engineers and service corps.
With what looks like a maker's mark and an arrow C below it, it looks Canadian made (or atleast Canadian owned) which would imply latter in the war '16 onwards but this is outside my comfort zone. The small narrow stamp is probably a town name rather than a date and name. Dates are usually after a name or on a seperate line.
Regards the issue stamps, apart from the obvious RCA, (yes that must be R' C' Artillery). I cannot help you. My experience is mostly with Australian issued examples of these and these will be marked up to "Field Artillery Brigades" (not batteries) eg "7FAB AFA". With 2 or 3 FAB attached to a division and treated as a semi-mobile unit which will be detached to assist other Divisions for short durations, as necessary. I would expect that the 13A and 7C stamps are similar unit ownership stamps at an artillery brigade level (ie "A battery 13th Field Artillery Brigade CEF"). Why the old unit ownership stamp is not struck out - I do not understand. The serial numbers, I would expect these to be a Q-store record number rather than a soldier's individual number. That doesn't mean that they are not the soldier's number but I would consider this unlikely. Again why the old number is not struck out, again seems odd.
I hope others can be more helpful.
PS regards date of manufacture, although this equipment remained in limited use up through WW2, examples of the bandoliers manufactured after WW1 are unusual.
1937 to 1942 manufactured P03 belts, bayonet frogs & water bottle carriers are quite common but not the bandoliers, belt ammo pouches or messtin covers and I have never seen a post-WW1 P03 haversack or greatcoat carrier.
When the leather for P03 is WW2 manufactured, the colour is usually a bit more orange than the WW1 dated ones(when showing original colour) of course both go dark brown with excess wear and saddle soaps and black if treated with neatsfoot oil.