Gunners, what am I actually looking at here?

All that Beamish had was a label, "17pdr SP Mtg, fitted with 25 pdr".

 

Sorry, Chris, I meant the second photo mounting the not-quite-a-17-pr

 

Sorry idler, I didn't mean that in reply to your post.

This will sound a little odd but I am not sure at what date the muzzle brake on the 17-pounder was standardized? I have not read anything in the Tank Board etc proceedings about mounting the 75mm HV in an SP.

I know wikipedia is worth squat, but it says that the 17 pr Mk 2 replaced the muzzle brake with a counterweight but then the brake was added back on in March 1944 with the introduction of ADPS.

 

This does at least tell you that it wasn't a dedicated 25 pdr SP.