They cost more then belts but they are worth it! I cut a good belt off so I could install then. You will need needle nose pliers to help pull the tab through to lock the tab. I used 3/8' for my lathe and 1/2' for my drill press. They come in 5 different widths to fit your needs. You can never get the wrong length, you can add or remove links until correct. They work great cutting the belt sound and they are very easy to install with out having to take your machine apart. Other places also sell them but I get parts shipped UPS ground in one to two days and they offer a bigger range of widths. I got mine at McMaster Carr, I added the link below. So they replace the last two or three digits with some other symbol. A lot of people don't want to put the actual serial number up in print, either hard copy or internet. I don't know why people do this but it's common on vintage vehicles and high end (meaning expensive when made and expensive today) radios. The seller showed it as 0198# so he means it is 019800 to 019899. The serial number of the new 101.21400 you found listed for sale is not 198. Sears sold this model until 1956, after which they sold the 101.21400 (same as 618) and the serial numbers started over. In the 1939 catalog, the 101.07301 with a redesigned headstock and larger spindle with 1'-8 nose and 17/32' dia. It had split bronze sleeve bearings and a 3/4'-16 spindle nose with 3/8' diameter through hole. The Craftsman model 101.07300 appeared first (and only) in the 1938 catalog. But Sears didn't start selling them until 1957. The Timken bearing 612 and 618 with 1'-10 spindle nose were made as early as 1937 (earliest Atlas catalog that I've come across) and as late as about 1972. Atlas made three (or possibly four) major variants of the 6'.

The 101.21400 and Atlas 618 serial numbers don't have any relation. I don't blame you for not wanting to pull the spindle just to see the dates! However, based on only the two points and the known catalog dates (years), I would guess that yours was probably made in 1957. I went through all of the serial number and date databases I've come across and did not find a single valid date for a 101.21400.

Sears date code