"The middle 38 tooth went on without a problem. The outer ring 48 does not sit in the step down (?) of the spider of a Shimano 105."

The middle ring and the outer ring use the same bolt to attach them. The bolt passes through both rings and then is tightened. Also a dab of anti - sieze on the bolts threads before reassambly is a good thing. Does this help?

Vin