300Gr RWS KS (KegelSpitze) Cone Points & round nosed steel jacketed FMJ solids (by default).
You were also given the option to ask for 300Gr Winchester Silver Tips & flat nosed lubaloy jacketed FMJ solids.
I know this for a fact, because a very close friend actually ordered a Heym boxlock ejector side by side double rifle (chambered in .375 Holland & Holland Magnum) brand new in 1977.
These were the only 2 makes of .375 Holland & Holland Magnum ammunition which were commercially available back in those days (between 1973 and 1983). There was also Remington, but nobody liked to use them for regulating double rifles. Because the soft points (270Gr Core Lokts) and the solids (300Gr round nosed cupronickel jacketed FMJ) were of different weights.