I have owned 33 Pre-64 Mod. 70s in the past 36 yrs., of which 3 were .300 H&Hs, I currently own 10 plus two original H&H modified actions stripped from rifles, here are a few points.
1. The firm of Holland&Holland developed the belted case, but, based on work done in Germany and the .375 H&H was NOT the first cartridge or case that was belted, the .375 belted rimless nitro express was and there was a European round either coincident or slightly before this, the title escapes my memory at the moment.
2. The WSM cartridges ARE NOT based on the .404 Jeffrey, they are original with Winchester and are actually based on the experimental .345 Winchester autoloader case that they worked with for the US gov't, sometime after the First World War. this took it's dimensions from the .50-110 or .50-100 rimmed case used in the 1886 rifle and possibly in the 1885 as well, I can't remember. The .348 Win. is the better known offspring of this round, chambered in the Mod. 71, Merkel double rifles, custom bolt guns and so on. The wilddcat predecessors of the WSMs were the Gradle Express cartridges developed in the post war years by Roy Gradle of, I think, Seattle.
3. The RUM cartridges were developed here in Vancouver, B.C. during the '80s, I know the people who did it very well. There were divers wildcats similar to these made up for years using I.C.I.-Kynoch brass. Reminton simply named these, with minor changes to facilitate mass production, much the same as the .22-250 Rem. which should be Wotkyns-Neidner, etc.
4. Winchester made many variations in the Mod. 70, I have a .375H&H with factory dual crossbolts, have seen a .300H&H with them, had a .375 with a steel buttplate and have seen .300s with red Win. pads, they are quite common. I have even seen .264s with pads from the factory. I had a .270 FWT. withh a whiteline pad which was identical to that on some of the .264FWTs and I am 99% sure it was factory. So, they would make about anything a jobber wanted to order.....in the good, old days.
These are fine rifles, I have four minty .338 Alaskans and love them. I can shoot the .300H&H from an unsupported offhand position better than anything else. I hope to find another one to replace those I had to trade for my Merkel in 12-12x9.3-74r. I have shot so many .35-.6" groups with these that it is no longer a surprise to me. I honestly think that the 1950's Mod. 70s in .300H&H and the .270STDs may be the most accurate factory rifles ever built and I have owned and do own about every type of highhgrade postwar sporter made, including the rare stuff like Brno ZG-47s and 21-Hs.
Anyone who can find an unaltered one of these at a good price should buy it, a .300H&H is about the ultimate in class, cool, and they kill like a deathray without the recoil and blast of a .300 Roy.