Looks like the flash is the normal artillery colours as seen on my avatar (minus the arrse colours). we would need more info abour said gun, ser no of breech and barrel to make any trace. There are guys who could help you with this on here i am sure if you provide this
It might be a Sub letter, but more likely its the tac sign with unit id.
From duuring WW2 (possibly earlier) until about 1970 all units marked their vehicles with 'Tac Signs'. These were the corps colour patch with a code number. However, these code numbers were standard, for example IIRC the 3 field regts in a div arty in the 1960s always used 67, 68, 69. A divisional medium regt where it existed may have been 70. However, I think the number may have changed after WW2 or in the 1950s. These code numbers were given in 'Staff Duties in the Field'. After the 1970s the colour patches went and the number was painted onto the vehicle in a much smaller size, this was abolished about 1982 when the army went OPSEC crazy and banned battery badges from vehicles.
Note that pre-Abbot and from late 50s(?) each field regt had 2 btys 25-pdr and 1 bty 5.5.
WW2 RA tac sign codes are available on-line, can't remember the URL but try googling RA 1939-45, I think the domain is wayblueyonder or similar.
The only real way to find a gun's history is to find the Gun History docs, but I've no idea what happens to these when a gun is finally disposed of. My advice is to start by asking this question using a FoI request. Pse tell us the answer.