"Yet another source claimed that the squares of Campa, Mavic and
Stronglight (ISO) are smaller than Shimano and Suntour (JIS) with the
result that JIS cranks will move about 4.5 mm further in.

This may conflict with the end of the square and result in a loose fit
or braking stress. for ISO cranks on JIS squares there's less contact
area between crank and spindle, since the crank does not slip perfectly
on the spindle. The result again is a higher risk of braking the crank.

But assuming those 4.5 mm and 2 deg, ISO is about 0.3 mm smaller than
JIS (2 * sin 2 * 4.5 = 0.3)?
ISO cranks (Campagnolo, Stronglight, others) won't go as far onto J.I.S. spindles as they ideally should.

J.I.S. cranks (most Japanese models) may bottom out if installed on ISO spindles."

(From Sheldon Brown's Site)

I think I may have just answered your question. JIS standardized parts (Shimano) have a bigger taper on their spindles. According to Toe-Clip, Sugino RD's are built to ISO size.

I am so sick of JIS vs. ISO. To solve the problem, both you and I will have to buy the matching Sugino BB.

Bummer.