The reason for such old rifles shooting high may be other than a different load. I have myself observed it also on several commercial Mauser A type rifles. Alas, the Rigbys were almost completely made by Mauser to Rigby's design, see Jon Speed: Mauser Archive, page 31-42.
Today we are accustomed to take aim with the top of the front-sight bead seen at the same level as the top flats of the rear-sight. This wasn't so during the early 1900s: Many shooters then sighted with a "fine bead". They aimed with the tiny bead nestled into the very bottom of the V-notch, hardly visible. Many old rifles were sighted to use such an outmoded sight picture that would cause a modern rifle to shoot very low.