spicer U joint part #'s??

[sope]

Anybody understand the Spicer part #'s for U joints? I've never put much thought in to it but I was dealing with a partsman tonight that couldn't cross it. Anyway after I found it for him in the book. I measured the 1550 U joint. I thought it was related to size but I can't figure it out how 1550 has to do with the size. I know the bigger # the bigger the joint but it must make sense somehow.
Any one know?

 

[protechk5]

What joints are you looking for? D60 axle shaft joints? If so those are 1480's. The Spicer non grease-able ones are Spicer # 5-806 X. http://broncograveyard.com/bronco/i-11793N-dana-60-spicer-1480-series-axle-u-joint.html