The original starter relays are 3 post versus new relays are 4 post. I am not sure of the mapping. the new relays are mapped as follows (going clockwise) I (9:00) M (12:00) S (3:00) and B (6:00).
Do anyone know how these are mappped relative to the old 3 post style?
Thanks

 

Can't help you with the four to three conversion but FYI, an early Ford truck relay is the same as the bike relay only stronger and cheaper.

 

I would like to know that we are talking about the same relay, the larger one which fits under the battery box which the positive battery lead routes to, then the solenoid then the starter?

If so you have 2 lager posts for the positive cable and one or two smaller posts. if you are wondering about having 2 smaller posts rather than only one you are still good to go simply don't use the other post.
Not sure which one is not used the new relay I got had a note along with the part telling which post to ignore if only one wire were used.
you could simply hook up the main leads correctly and with a jumper wire or your switch wire try one if it doesn't excite the solenoid try the other.

 

on '76 shovel when rewiringi changed from a big 3 post to the small box 4 post. had to get a copy of the wiring diagram from a later model that used a 4 post and and look at that next to wiring diagram for '76 and you can see what goes where,it was over 10 years ago, forgot the exacts.