Your posts got me curious, Phil. I can't at all remember how my 122S exhaust used to mount so I went looking to see what you were talking about. A bit of sleuthing turned up a couple of interesting references to the 657783 bracket that you and/or Eric probably already found, but I'll mention them here anyway.

There's a source in Sweden https://www.cvi-automotive.se with a catalogue listing that part with a schematic diagram that shows what it looked like https://www.cvi-automotive.se/upload/PDFcatalogues/pv_eng.pdf -see page 15. Whether or not they ever actually had that bracket in stock or could supply one now is a whole other matter, but it probably wouldn't hurt to ask. I imagine fashioning your own would be simplest and with a bit of effort cutting the profile angles could be made to look fairly stock. If you use a torch to help make the bend, a bit of extra braising as reinforcement probably wouldn't be a bad idea.

There's also mention of that part here in this old Volvo Service Bulletin on the UK Volvo Club site https://www.volvoclub.org.uk/tsb55-60/Group602-NoA4.pdf with what looks like a slightly newer style mounting used on the later 120s that may be of slight interest to you. That must be the B18 mounting you're talking about. If those parts are more readily available, it strikes me it may be worth the effort to weld the mounting if you're going to be doing a bit of driving. Nicest thing about that mounting is the rubber bushings as a vibration and torque absorber.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now