To the best of knowledge, Fender never offered an 18-inch speaker in the 60's. The original Showman used one 15-inch JBL D-130F in a unique bass reflex cabinet design where the speaker is mounted in a sheet metal ring which was bolted to a panel behind the front baffle board. The bass reflex port is concentric with the 15-inch speaker. (I know this, since I owned one of those cabs.) Viewed from the front of the cabinet, it looks like the cutout for the speaker is much larger than 15 inches, which it is. The driver however is a 15-inch unit.
edit: Sorry Bill, I just saw your post. I had a black "single" Showman cab that had the "tone ring." It was a pretty unusual way to make a bass reflex box.