Machine dating and what to hang on to...

[MachinistCarpenter]

Hey everyone. In the last 3 weeks I have acquired 6 complete ss's. The serial numbers are as follows....
33918 1979
331556 1956
159303 1983
1060 1973
14903 ?
31591 1979

What is the 14903?
Are the other dates I have come up with right?
Which ones should I hang on to? The first 2 need minimal tlc. The last three were near mint. I just took their tubes out and polished between centers in one of my metal lathes. I am ordering a few new bearings here and there. I will be reconditioning the motors as well. I'm a machinist in a electric motor repairs shop, so I will completely reconditioned entire fleet. I don't fell that I need all 6, although I'm madly in love with these beauties... Lol, which ones should I cut loose?

[bobgroh]

Although far from an expert, here is my gut reaction:
1. Keep a 520 over a 510 over a 500
2. Keep anything built after 1986 over earlier models
3. Newer over older
4. Condition obviously weigh's in but, if all more or less in the same condition - I would go with 1 through 3 above

FYI: I purchased my first SS (a Mark V model 500) from SS in 1986. Purchased a used 510 in mid-1990's and then purchased another used 510 a year or so later (primarily to get the jointer that came with it). Sold the 500 to my son-in-law. Upgraded one of the 510's to a 520 with parts from eBay circa 2004 and then used the 510 parts to upgrade my S-I-Ls 500 to a 510.