Rick ,
I don't have the bike here to measure the spacer , but , as I recall it's about 3/4" long and fairly heavy wall type material .
I don't see it listed in the M manual . The bolt seemed as original as the others , as did the spacer . I'm inclined to think it came from Simplex the way I bought it some 49 years ago .
Will ,
Yes . It's the original cover . Here's a cleaned version :
Pete .