photo of xx43 unknown loco or date

At the Cardi-Bach museum we have several hundred photographs donated by members of the public. One of these photos show the fireman and driver of a GWR pannier tank.
We are trying to determine the number of the loco - possibly an 850 class(?) converted from ST to PT note the extra 'bulge' on the washer cover to accomodate the motion, seen on pics of 850.
I've searched the GWR website, and Pannier Tanks, plus the BR website.
Could be 1743 or 1943. Any ideas, everyone?