It's a Musgrave single shot target action. They were made in South Africa, and starting in the 1970s they were one of the best target rifle actions on the market. Nowadays they are a bit "dated" when look at next to the new actions, but they are still every bit as good as they always have been. I have one, which I have shot competitively in F-Class (scoped) and TR (iron sighted) competition over the past 15 years.
On the left side of the receiver it might say "Lyttleton RSA 7.62" (mine does). Lyttleton is a suburb of Johannesburg, RSA is Republic of South Africa.
The receivers were not made with serial numbers, rather, the serial numbers were put on the barrel (in South Africa, the barrel is the registered "firearm" part, the receiver is just a miscellaneous part).
I am a bit surprised to hear that it is in 300 H & H. The action would have originally been built for .308. Conversion to .308 would require opening up the bolt face. From the pictures you posted, perhaps I can see that?
If I had to guess, I'd say somebody built themselves a hunting rifle, using target rifle parts and practices.