You should be OK but……………..
K6642 would be a very early ML7 bed no later than 1953-4 which, if you cannot inspect it, may well be worse than your existing setup. The S7 shared the same bed as the ML7 from its introduction, but there were variations later on. Why not play safe and get a bed regrind of your existing machine and hence know that you have a machine you can trust?
Would you like to put us more in the picture? Can you inspect the other bed or are you buying blind? Is your present bed truly defunct as in bent or badly bashed about? What is the state of the rest of the machine?
Dennis