Mr. De Haas, in Bolt Action Rifles, his book, has a standard I subscribe to. More for 98's but not bad advice. Pull the wood off and look hard under the front receiver ring. Should be able to find a proof mark with date. For more modern cartridges like .243 dates after 1920, 1930 to be extra careful in the '98.
b). 7.62x39 will be loaded to varying pressures depending on the source of the ammo. It is not the mid pressure cartridge of the early 1900's like 7x57, .303 Brit, etc. I would want a later dated action.
c). 7.62x39 is of smaller diameter than the Mauser standard. You will need an extractor adapted and magazine, thus the kit... AND the smaller diameter means that there will be less pressure exerted to the rear on the bolt face... simple physics. Reverse of the Contender handles the .223 at 55K CUP or so, but not the .308 at same pressures. This should also buy you some safety.
Assuming you are willing to work with these bounds, go for it. Real decent guns. Keep them working. Luck.
