I see the hesitancy from posters for avoiding the 135W 35mm pano backs for both the Bronica SQ-A, SQ-B and ETR bodies.
Prices seem high... but they do come up on eBay more often than one may think.. albeit at around $300.
I looked for such a back for a long time, and passed up many based on price. However, I wanted to shoot some 35mm pano.
So, I took a look at options... WOW... the Bronica backs are cheap by comparison.
The two camera's that come most readily into the discussion are the Hasselblad Xpan and it's manufacturer's own Fujifilm TX-1 (built and branded for Hasselblad).
Sold prices listed for many of these two range from $1200 to $3000. Athough these shoot 24x65mm, the price makes any Bronica 135W back I have found, paltry by comparison.
Another option is the very hard to find 24X65mm 35mm pano film back for the Hasselblad 6x6 CM. There is currently one on eBay running at $1200. Again, pricey.
So, even though the pano is 10mm shorter for the Bronica backs, I suspect another plus is that shots are through medium format glass, using the center portion of the lens.
Once, I made those distinctions, I revisited the Bronica 135W pano listings on eBay, and within 30 days, I picked up two (one SQ and one ETR) backs, BOTH for $425 shipped (NOT EACH, both). I have the SQ back and it looks unused, with manual. The ETR back is on it's way and should be here Monday.
So, I am anxious to get out and try these out. While the ETR back is regarded as fiddly, nothing can be fiddlier than loading any film in the inserts of many Medium Format camera's, and particularly into an FSU Kiev (early edition).
I now feel the prices for the Bronica backs are somewhat a "steal" compared to the hasselblad/Fuji options.
There may be other options (doesn't Holga have a 135 pano camera?) but these were all I needed to loosen my wallet strings.
Worst case scenario, I like one and sell the other for over half my outlay. Huh?
I'm thinking the SQ will be better on the loading, and better on the portion of glass used for a lens optimal for 6X6. The one for the SQ also has exposure comp setting control for the electronics on the SQ. Not so the ETR.. but that's minor.