So ~~2y ago, I got from Jordan, one good strut insert (Bilstein econo) and one full assembly.

Slow week at work, brought the car in and the gland nut (gold nut top of strut internally) needs to match the type of shock I have.
I am putting this in a 1982 245 naturally aspirated, so.... that there is a difference... has me concluding the inserts I have are for turbo.
So by Volvo I need 1272231:
https://www.volvopartswebstore.com/products/1107001/1272231.html

So, years ago these were easily sourced. Now, not so much.

Apparently this has more to do with brand of shock insert. Some step down the insert body at the top, some do not.
The various manufacturers may have copied multiple Volvo styles, but Napa list a value/domestic brand that has no step, yet all Sachs appear to have steps. Kind of tells me this isn't a sign of whatdaaycall it single wall design.

The Bilstein Touring (entry level) do not have the step, whereas the tired out, lemon yellow (not Bilstein, perhaps Boge) ones previously in the car have a step.
Distribution of these parts is: normally you pull the new insert out of the box and the correct nut is supplied with it.

I talked to Bilstein and aside from my having to go through a distributor, they definitely have the nuts on hand. b6-sc5-z001a20 No mention of price, but could be less than $10US.

Essex on the coast is looking into it for me, after a very pleasant helpful call.