Large calibre cartridge cases during WWII seem only to have been made at the South African Mint, of which there were four locations: Pretoria, Kimberley, Ladysmith and Johannesburg. Pretoria was the primary site, making complete rounds of 2-pr (A/T), 6-pr (A/T), 13-pr, and 18-pr; and cases and primers for 25-pr, 3.7-inch howitzer and 4.5-inch howitzer. The attached example page is of 1942 Ammunition Production Deliveries from the SA Mint, Pretoria, to 1st July 1942, which gives some idea of the ammunition on order, being made, and in what numbers. (The eye-straining image is due to the original document being a poor carbon copy, rather than my inept photography, but hopefully most details can be deciphered.)
Of the other sites, Kimberley was an extension of the SAA facility at Pretoria, and Ladysmith was the main facility for reconditioning and reloading of fired cases - particularly 25-pr for India from 1944.
Since the head stamps of the four cases shown in the opening post show no markings of having been repaired and refilled, it is probably unlikely that the A in a diamond is an annealing stamp, but more a final inspection stamp. Given the consistent position of the U at the 3 o'clock manufacturer's monogram location (on British cases), then I would say - speculatively - that the U indicates manufacture of all four cases by South African Mint, Pretoria.
Tom.