Numbering is inside circumference. SC is "Square Cut". Not sure what the X is, but I just order belts labeled "small square" or "square cut" and match the circumference. I would probably order a 4.2 or 4.3, but see what the other members have to say before ordering.

Another weird symptom has cropped up, when I power the player on, it seems to jam immediately and then eject the CD tray rather than the main one, despite direct CD not being on. Figure I should regrease the rails too?

Clean and re-grease anything you don't have to disassemble with white lithium grease (tube not spray).

Not sure about the CD tray since I can't recall this loading assembly off the top of my head. Have you removed the tray or messed with the gears by chance?

Apart from having to move the laser assembly to actually remove the pickup belt, no.

Well I did get a replacement belt (the stock SCX4.6 one), but it didn't seem to make a lick of difference - but I rubbed the load belt with some warm water and wiped the pulleys with alcohol and that seemed to fix things up, so I'm guessing at this point the load belt really was the source of my troubles all along, but it remains to be seen if this fix finally sticks.

Good to hear. You said you replaced the loading belt not too long ago? It might be ever so slightly too large if so.

When my CLD-D925 hasn't been used for a while, this symptom of yours actually happens for me and I get the U1 error. The U1 error code is related to "miss clamp", so a belt exchange could solve your problem.
I remember a thread where Krbahr has mentioned for a forum member a smaller belt size. Check the second post of this thread in the link. Might be that your belt is still too large.

Probably, because the problem started occurring again, after about two weeks, although it's not as bad as before, but still.

I actually tested the player by replacing the belt temporarily with a much smaller rubber band I had lying around, and although sluggish, it actually loaded and played the disc properly, so clearly all I gotta do is replace the belt with one that's slightly smaller spec. Question is, which belt should I get?

Well that fun lasted for about two days. Now I can't get the discs to load at all, again. I already ordered a new belt, this one's a SCQ3.6, which is smaller than the spec belt, but blam1 suggested it.

In one hand, I feel like I should try greasing the gears or something, but considering I was able to get the disc to load by moving the pulley with a fingertip while the cover was off, it definitely feels like it's just an issue with slippage and maybe that belt I bought in the first place wasn't all that good to begin with.