I have an original SL66 and an SL66SE. My understanding is that there are three series of backs. I'm listing only the 6x6 ones but there are some 6x4.5 (horizontal) ones but i think they still only give 12 exposures per roll.

1. Original SL66 back. 120 or 220 according to a switch. Film loading operates by a feeler which is usually reliable but can be fooled by thin films and can need adjustment.

2. SL66E back. I don't know if there are separate versions for 120 and 220. Film loading by feeler as above. Film speed setting for meter.

3. SL66SE back. Separate versions for 120 and 220. Film loading by match the arrows, so immune to getting out of adjustment. Film speed setting for meter. Easily identified by having a fold-out winding handle instead of the pop-out rotating knob. I think this is the best, unless you need to use 220 film.

In the troubleshooting guide of the SL66SE manual, a cause of the meter not working is using a magazine other than a SL66SE/X magazine. It implies that the camera would work but without the meter. I don't know if the SL66X magazine is a different variant. It doesn't say that using an SL66E meter is a fault specifically, but it doesn't specify it as a solution to the meter not working.

In the troubleshooting guide of the SL66E manual, a cause of the meter not working is using a magazine other than an SL66E magazine. The SL66E was earlier than the SE, so no mention of the SE magazine.