To answer your last question first, by default, this is the accepted thread in which to report new (to you) machines, serial numbers, bed lengths, etc. Although there are plenty of examples to the contrary, there should be only a few posts here regarding a single machine., Questions about it and the restoration tale belong in the regular A/C threads.
If you aren't certain whether or not you reported your 12", just post the serial number, model number and bed length (only the 10" include the bed length in the model number). I'll let you know whether or not it was reported.
Back to your new to you 618, unfortunately neither Atlas nor Timken ever engraved the bearing inspection dates on the bearings in the 6" lathes, mills or shapers. So absent any other info (such as original invoice, Grandpa's memory, etc), all that we can do about dating those machines is an educated guess assuming Summer of 1936 to Summer of 1973 as the span of production for the 612 and 618, constant production rates and something more than the highest known serial number (currently 27421) for the total production. Anyway, subject to those caveat's, your 618 would have been made around July of 1944.