not strictly true marc

yes, they will bolt onto a sierra rear disc axle as the bolt pattern is the same, but they aren't the same as the xr as they only fitted vented rear discs (and matching calipers) to the 4wd cosworth int eh sierra range

however, finding the correct discs for suitability may be the issue, as, from memory, you'd need to get them for a scorpio frogeye estate which uses 253mm vented discs

if you can however source some cosworth spec rear hub flanges, then you can use these to fit 4wd sierra/escos rear discs that are 273mm and vented

the granada scorpio 24v (5 stud) and the scorpio frogeyed 24v (4 stud) used the same calipers and disc size as the cosworth 4wd cars @ 273mm vented whereas the granada/scorpio estates used vented rear discs but in the smaller 253mm sizes