You got me curious as I don't recall retractors in the few old 240s I've seen with a rear facing 3rd seat. Doing a bit of a search for you, I see 3rd row seat parts for sale (like on ebay and www.2040-parts.com) and as well Genuine Volvo parts schematics (like on www.volvopartswebstore.com) for 3rd row seats with and without shoulder seat belt retractors. I can't tell what year they were introduced, probably in the mid-1980s, but I imagine they were introduced in all markets more or less at the same time as the body required a lower attachment point and all bodies came from the same Swedish plants. When a car was ordered with 3rd seats as a factory option you would have received retractors as soon as they became available. However, when ordered as a dealer installed accessory there would have been a mix of old and new stock in the parts chain unless Volvo mandated a substitution so that may explain differences in what you see installed.
The retractors attach low on the outside, just forward of the rear seat belt retractors. In your 1993 240, if you remove the side panel (at the risk of breaking the odd plastic stud) you'll see the threaded bolt hole. For people with early 240s buying a used 3rd row seat that has retractable belts, you should first check behind the side panel to see if you have the needed attachment point as it may be difficult to add your own to safety standards.
Note that Volvo calls these "extra seats", so when google searching say
240 seat extra OR third OR 3rd
(use the upcased OR to explicitly indicate equal weighted keywords)
Example photo with retractors (1991 245)

Example schematic without retractors

I wondered if a set of retractable belts from a 740 or 940 would fit a 240 as the rear area is similarly dimensioned and the attachment points are also similar, but the retractor reel is located behind the side panel so will not have a cover.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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