A10 plunger oil tank question

[chaz]

the A10 plunger has a oil tank fitted thats not looking like a standard one. filter is at the back , parts book shows at the side.
every time I have to drain sump (seperate thread) , I have to undo the filler cap with pipe grips.
Ive turned up custom filler caps for the Triumph and A50's , anyone know what the tank is/was from , also thread of cap as its at home with lathe but thread gauges are at unit.