Seems you found one of my image compilations. Top portion is SAAMI. Bottom is from a UKicon military blueprint of the SMLE MkIII*. Not really much difference between the two - once one subtracts headspace from the SAAMI lengths.

Despite tales of wide variation, I find no significant difference between chambers of my 1943 Lithgowicon SMLE III* and 1953 ROF(F) No.4 Mk.2. Fired cases interchange perfectly and shoot the same loads with comparable accuracy. Fired cases also interchange in my ROF(F) No.5 Mk.1 - but accuracy isn't on par.

Are you planning to make a chamber reamer?