Of the 1500 No4 trials rifles converted to snipers

[breakeyp]

I got one. It has ROF 41 20908A on the socket.
Was not a sniper though.

You have a special serial number rifle that is not normally seen. The A suffix indicates the gun was made with non standard parts and was to alert the repairman that extra effort was required to fix it. The trials guns under discussion were only made in the early 1930's to 1935 and were serial numbered with a A prefix. You gun is a little late.