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A further bit of digging shows the Duralast C8426 you got from Autozone is indeed a remanned ATE right rear caliper. The fitments of 240 models and years (and some 160/260 models) listed on the Autozone part page appear to be accurate and will fit your 1993 240.
Your non-ABS to ABS brake question is well taken. The rear calipers, rotors and pads are about the only things that didn't change. Here's the story as best I can determine and recall.
The introduction of ABS on our RWD Volvos varied by model and market. First available in 1984 as an option on some 740 models, in North America ABS became standard on 740 GLE/GLT models around 1989 and all 240/740 models by about 1991 (I'm sure there are many exceptions to that).
To implement ABS, Volvo had to abandon their innovative anti-failure independent split triangular brake design. It used a dual circuit front caliper. Each circuit linked one of the two circuits (piston pairs) in each front caliper together with one of the single circuit rear calipers, either the left or the right. For ABS, Volvo now went with the more common independent front/rear design using single circuit calipers all around. The front calipers were of the now common single piston floating clam shell design. The system had to have a different distribution block (w/pressure loss sensor as before) and they dropped the two rear proportioning valves in favor of an integrating single pressure reducing valve in the distribution block at the front. The master cylinder changed for ABS, but the brake booster did not. Note that different boosters were used on the later B230FT turbo engines having a more powerful vacuum pump.
The ABS control unit uses speed sensors on reluctor rings at both front calipers, but rather than going with a fully independent 4-way ABS system, Volvo decided to go with a 3-way ABS system with both rears on the same ABS circuit meaning the rear speed sensor was put on the drive line (transmission output shaft) rather than at a rear caliper. Thinking about your caliper question, I thought they might have decided to beef up the rear braking with a larger rotor or piston (a 38mm bore) at the same time, but that was not the case, they kept the same rear caliper and mounting as before.
Checking the fitment list on an actual Volvo parts page, the same rear calipers were used on all 240 and 260 models with ATE rear brakes right from 1976 through to 1993. All 240s/260s after about 1980 got ATE rears, but prior to that you had to check whether you had Girling or ATE rear calipers either by inspecting the caliper or decoding the vehicle product plate. Among the various Volvo part numbers you'll see for 240 ATE rear calipers are 1229549 left and 1229550 right. These are different from the ATE rear calipers used on 740 and 940 models, although they all use the same pads. Note that although the earlier Girling rear pads are very similar in size and looks to the ATE rear pads, they are slightly different in terms of pin slots and height.
Hope that extra information helps further clarify things for others. Others can chime in with additional info and refinements to what I've said.
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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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