Ted is the guy that owns 7.62x54r.net. He's a pretty good guy. I know him from another forum.
Snipers did not, at that time, use different ammo. If you look at that chart, the 7N1 sniper ammo did not come into use until 1967. It was all either Type L or D. Lead core light or heavy ball, that's it.
As for modern weapons that are 'more efficient,' what are you talking about? There is nothing magical about a Glock or a SIG-Sauer that makes them 'more efficient' than a P08 Luger or a Walter P38. They're still firing the same 9mm round. The difference in manufacturing processes and materials, mechanical reliability, etc., doesn't make the weapon necessarily any more deadly.* Old technology doesn't necessarily mean bad tech. Look at the M1911, for instance.
(* Okay, of course if the gun jams then it's not deadly, and the P08 did have a habit of jamming. But the round itself isn't any different just because it was fired from a Glock instead of a P08.)
(That said, however, there are some things that were just never a good idea in the first place and were superseded because they sucked so hard. The 7.62x35 round for the Nagant revolver, for instance. Actually, the whole Nagant revolver in the first place. Neat concept, but it was the answer to a question that nobody ever asked.)
Also, this 9mm incendiary ammo you see advertised is sort of pointless. It's a novelty item, and not really worth spending money on. It's not going to be any more deadly, and in fact isn't likely to ignite on impact with a person.
Why would they? Belts weren't really loaded in the field, and I highly doubt that they'd be reloading the DP-28 drums during action.