Your proof marks appear correct for a pre 1904 nitro rifle.

In my Holland 500/450, I use 87.5grs of R15 with the 480gr Woodleigh soft for 2060fps and 85.5 grs with the Woodleigh 480gr solid for 2090fps. Horneber brass, Kynoch foam fillers and Fed 215 primers.

I also shoot Hornady 350gr RN with the same 87.5 gr powder charge. This load shoots to the same poi as the Woodleigh 480s and makes a fantastic practice/deer/pig load.

I hope you enjoy your rifle.