More early AMG IDs

Following the list from ZC to ZZ, something more can be added about some of the models meant:

ZL, Bob McCune 

ZS, Leonard Chambers

ZU, Wally Shillcutt 

For YA-YZ:

YH, Keith Stephan

YI, Miles Conley 

YS, Lars Anderson 

For XA-XS:

XH, Steve Wengryn