There are four different dark slides for Bronica backs. These four slides fit the eight versions of backs as follows:

1. Deluxe - fits only Deluxe backs (photo)
2. S and early S2 - fits all 120 only S backs and 120/220 S2 backs with curtain (photo)
3. S2 (late model) and S2A - fits 120/220 S2 backs without curtain and all S2A backs (photo)
4. EC - fits EC backs only (photo)

Deluxe - fits all Deluxe backs (2 types). These are only backs with opening latch on left side instead of top.
Discontinued - no product number available

NOTE: There was a second Deluxe dark slide with the same dimensions but with a short, square handle. This could be folded flat against the slide to prevent it being accidently pushed in and releasing the back.

S and early S2 - early S backs are 120 only - havea no 12/24 exposure switch. Later S and early S2 backs (CM81500 or lower serial number) have an extra curtain in front of the film plane, as well as 120/220 capability.
Product number 83023

S2 (later models) and S2A - later model S2 backs do not have the extra safety curtain (serial number CM81501 and up) - except for very minor differences, S2 backs above serial 81501 and S2A backs are identical.
Product number 83024

EC - fits EC backs only. EC backs have double button release on top and A/D switch on right side.
Product number 82029

Film back curtain - the only way the safety curtain is visible on early Bronica backs is when a slide is in the back. If the slide is partially withdrawn while the back is off the camera, the metal edge of the curtain is visible on the front edge of the slide. If the slide is fully inserted, open the back and remove the insert and the curtain will be visible.