The first 020 saws were 1114 series, all mag housings, in both a top handle and rear handle version.
1114 020 1971-1979
020AVP 1972-1979
020 SUPER 1979-1994
It then became an 1129 series. It is available as a rear handle as well. This is the saw of choice for most tree guys.
1129 020T 1994-2001
MS200T 2001-current
The most common problem with 1129 saws is that the anti-vibe buffers get torn, and then add to that a dull chain, and the handle flexes up too far and tears the intake boot, so it may idle but will start sucking gobs of air and die if the handle is moved at all. Be sure to check for that by lifting the handle up while pushing down on the body and see if the boot looks OK, and that the rear mount is OK. There is an upgrade kit for the tank vent for older saws, and the most current have a Nikkel vent up under the handle housing, with a different tank as well. It will have a double fuel hose grommet, one line to the carb and the other to the vent.
I have one I just repaired with a new mounts, boot, vent, and carb, and it runs great and has plenty of life left in it, and is absolutely worth 225.