Sorry, the different size I showed was my error. The correct bolt, whether for an '85, an '89, or any other Volvo 240, is as you found, M12 x 35mm long with a thread pitch of 1.75.
I think your Volvo dealer could find this bolt if he tried a bit harder. The new rotors are still being shipped with them. I learned from one dealership parts man that all the older RWD parts were cleared out of their inventory to be ordered from Volvo GCP (Genuine Classic Parts) or some such separate company if necessary.
It is interesting to see the images offered by eeuroparts, ipd, skandix, etc. when searching on 975546, show flanged bolts with no unthreaded portion or threadlocker. The bolts shown below are what you will likely find on your car, and what come new with genuine Volvo rotors.

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Art Benstein near Baltimore
"Never trust the work of the last guy, even if you're the last guy"
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