It turns out that I did not need the spacer part! I have come to the conclusion that this is because my Strida never had this part in the first place. This is interesting because the diagram in the manual clearly shows that this part (338) is there. I suspect that this is a key difference between the 5.0 (manual) and the 5.2 (my bike). Am I right?
When I tried to install the part as shown in the explosion diagram, it did not fit correctly. It fit through the magnet (part 336), but it did not fit into the axle. The axle on my Strida has a tapered extension that is approximately the same diameter and length as the spacer part. Also, it does not seem to have part 449. So, when I tried screwing the magnet back into the axle with the spacer in place, the spacer hit against the axle, and the screw could not go far enough into the axle. Therefore, there was a large gap (ca. 2 cm) between the magnet and the wheel, and the wheel was not securely mounted – it could slide along the axle.
So, what was the cause of my problem? You gave me the important clue. You said that with the new alloy freewheel, the spacer (340-06) between the brake disc and the freewheel was not necessary. I thought this meant that the freewheel spacer was optional, but in fact you must remove it. I removed this spacer and discovered that my wheel fit properly again!
(Another important difference to the 5.0 is that my 5.2 has 3 washers, 1 plastic and 2 metal, on the outside of the magnet.)
Anyway, hope this information will help others who are installing the alloy freewheel!