The service manual says that the ball bearing for the SLT12 is .281"
It also says that the bearing well should have about 3/4cc of"Moly Spindle Oil No. 95" from Allube Corporation of America.I did a google search. Apparently, Allube no longer exists.
I'm thinking about using ISO 22 Moly Spindle Oil from Schaeffer at www.schaefferoil.com.
I ordered a service (repair) manual from Radio Era Archives in Dallas Texas. I also ordered an owners manual from Audio Classics.
The cartridge in the SLT 12 is a Shure V15II. Stylus for this cartridge may be ordered fron Jico in Japan (www.export-japan.com). They also sell a hyper-elliptical stylus for the V15II. I think I may use their stylus rather than trying to wrest the current cartridge free from the headshell. I do not think it will come off without damage.
I don't know what harm the magnet in the arm housing would cause. I plan to get my turntable up and running, then try the turntable with and without the housing to see whether I am able to hear the difference.
Good Luck