Bronica Lens Serial Numbers

Bosch kts 520 drivers. All Small bayonet mount Bronica Nikkor lenses fit the S body. No external changes, the model is identified by the designation S2A following the serial number.

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I just received a classic Bronica today picked up from a certain online auction site, but my research so far is giving my contrary indications about whether I have an S2 or an S2a. It was advertised as an S2 and I bought it on that understanding though I'd be a little happier if it turned out to be an S2a. The serial number is in the 120000's and lacks the S2a extension which indicates this should be an S2.
But the winding knob which I understand is the only visible distinguishing feature between the two versions appears to be the S2a style.
Any theories about the contradictory identifiers or have I just misunderstood something?
Also, as was noted in the description, the back won't come off. Pressing in on the dark slide seems to cause something to begin releasing but only barely. I understand there are two styles of dark slide for these cameras. The one I have has a curved leading edge. I was wondering if it was possible the problem is that I have the wrong style and if so is there any way to tell? Alternatively is there any other common cause for this issue. I can get by without removing the back for now but eventually I hope to get a second back so I can switch film as the situation dictates.