Yes.....there is some confusion here Steve....but I think perhaps you are half right...but half wrong....
I do think that.....from the factory....there were THREE different types of rear struts
Type A - used on all 1300 X1/9s worldwide. Yes...these had the "straight" lower bracket. And on the upper shock shaft a 14mm shank portion for the aluminum cone and pivot.
Type B - used on "USA": spec 1500 x1/9s. These had an "angled" lower bracket to provide clearance for the 1500 driveshaft flange. although some say the 1300 bracket has just barely enough clearance to be used. Although I guess not quite enough clearance for the fiat engineers to be comfortable. In addition, at this same time......Fiat re-designed the upper spring perch - I assume as a cost-saving measure - to eliminate the aluminum cone and pivot and washer and seal they used on the 1300 cars. As the rear struts do not need to pivot like the front steering struts do. So the upper shank of the shock shaft is different. You can see that in the pic above.
I DO think the spring perch was in the same - or very very close - place ...on both these struts. And resulted in the same ride height. and can be interchanged if you use the matching upper perch and hardware and bits.
Type C - used on "Euro"' spec 1500 X1/9s. These were the same as Type B...BUT...had the bottom spring perch on the strut....moved up about 30mm or so. Resulting in a HIGHER ride height on the rear. Yes...this seems very odd...as one would think the raised perch would have been used instead on the "USA" cars...to meet bumper height regulations...and to counteract the increased rear weight of the USA EFI bits, converter, A/C bits......
It seems the same rear springs were used on all above....
It would seem the above even confused Koni.......Best I can tell....Koni - being a euro company - were unaware of the Type B strut. and only made Type A and Type C. See the pics below from my old Koni catalog.
View attachment 78242
View attachment 78243
Which seemed to result in USA folks fitting the 87-2171 rear Konis to their 1500 X1/9 finding their rear ride height now about 1" higher...and scratching their heads.......which Steve you seem to agree with........As far as I can tell, Koni never made a Type B rear strut.......
As for other available rear 1500 struts....it seems that indeed KYB figured it out and offer a Type B rear strut ( #233004 ) here in the USA. Note they offer different rear struts in the euro market ( assume Type C ). Eurosport UK seem to have it all figured out correctly and offer all Type A, B , and C rear struts. Those here who have ordered other makes of rear struts from europe have often found they were supplied Type C struts with a resulting increased ride height......
I still reckon that ALL stock Fiat supplied USA rear struts - 1300 or 1500 - result in the same - or VERY VERY close - ride height. As shown in my initial pic in post #16. Guess we will have to agree to disagree about that