I checked the invoice, it states "CR 243 joint 24 mm x 63,1 mm".
My searches cannot find anything that looks like "CR 243", but CR did manufacture u-joints before being absorbed by SKF, so maybe I've been sold a pair of NOS joints.
The dimensions did lead me somewhere though, and we may be able to settle this once and for all.
They first lead me to the GMB u-joint catalog which lists two sizes of u-joints for 101 and 105 Alfas: earlier 1300-1600s up to a fuzzy point before 1750s had 15/16 (0,938'' = 23,825 mm) bearings, and later cars had 24 mm bearings (0,945''). The width of the earlier bearings is 61,3 mm (2,413'') and the later ones 63,2 mm (2,488''). Early bearings will slide in later yokes with perhaps a little radial play, but the axial play from the insufficient width will be a problem for sure. These dimensions are exactly what I was dealing with.
GMB's 24 mm joint is part number 220-0410 and fits Alfas only. Their part number for the 15/16 joint is 220-0500 and it fits a wide variety of other cars, including early Volvos. I had been told by an older Alfa owner that Volvo joints could be used on an Alfa - partly true.
I then found other u-joint catalogs which quote only one part number for all 101 and 105 Alfas (Neapco 1-0300, Spicer 5-101X) but both of these are the shorter 15/16 joint. This is the source of the problem: bad application guides that fail to identify the two variations.
Rockauto currently lists the correct GMB 220-0410 u-joint for 2000 Alfas ($15), and also Moog 413, SKF UJ413, and AC Delco 45U0010, and from the quoted dimensions these are all correct .
Could it be that Spica supplied the springs to take up the slack between the two applications and make the replacement joints "universal"? In the early days of our cars, it seems than Alfa did not list u-joints, sold new drive shafts instead.