[PlasmaKrab]

Just add the early M-60 variants in the same unit class as the other earlier tanks (like the M-48) and give them an X3 radio code.
That will prevent them from being all over the place when the IA buys units, but they will be available for scenario design etc.
If you have already tinkered with the unit information tags (I haven't yet), you can add a little something saying that "the M-60s were mainly deployed to CONUS and Europe due the higher Soviet armoured threat, etc., but could have been shipped to SEA at short notice to counter an unexpected NVA armored push" or whatever. Not that it makes much sense now that I write it, but hey, it gives you a reason to keep it on board.