@Frederik Badenhorst Cartridges of the World (COTW) 3rd ed. (1972), 6th ed. (1989) and 12th ed. (2009) all loading data, as follows:
125gr 30 grains of 4198, MV 2370, ME 1560; and
150gr 41 grains of 4064, MV 2350, ME 1840.
The use of .316” - .318” diameter bullets is recommended.
The 150gr is comparable to the 158gr factory load listed at 2380 fps and 1840 ME and would appear to be developing similar pressure to the factory 196gr load listed at 2,099 fps. The 157gr factory load listed at 2155 fps may have been a lighter load developed for the 8x51 R round.
You appear to have two choices: firstly, to continue your historical research by looking for books/articles published primarily between 1930 and 1960 and/or;
secondly, to work with someone who uses the Load From a Disk software package in order to calculate a suitable starting load.
I suspect that either H4350 or a close equivalent would be the most suitable propellant. Best of luck.