Ok the tag for a sickle unist is on the front top side of the mule drive. I can see a tag in the picture, but some of the mower units also had one there. The drive for the sickle is differnt than the one for the mower., but it would probally work. As far i know none of the sickles have there tags on the sickle unit itself, and only some of the mowing decks have theres on the deck.
Yes the mule drive is for a rotary mower 60655. I have a second 60656 sickle with the correct (tagged) mule drive and yes the mule pulleys and belt guard are different. I know the later sickles have a tag on the frame but those are 917.25120 sickles.
I would like to find the differences in the older sickles so I could identify one in case I ran across and older 60639 one someday.