Here's a blast from the past:
http://hv.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=007zH9BTW.... the answer to the question is NO, a 1975 CB500T seat does not fit 1974 CB550. Actually, it will sit on it. It will rest on it, but the hinges will not bolt properly. The edges of the seat will rest on top of the 550 side covers and scratch indentations in the 550 side covers.
Actually, it is a 2 part answer...
Hope, you don't have a 74 550. You may have the bike registered in the state of Louisiana as a 74 550 because the frame says 550, but the VIN on the frame indicates that you have a 75 550. Also, the paint job on the tank indicates that you have a 75 550.
Oh, I've learned SOOOOOO much since then!!!!!