I’d be on my way to check them out if I was close. They’re as tall as that stool and the opening in the baffle is huge. I’d say they are “18 inch and maybe a 1 in a 100 chance that they are 21 inch speakers. Fun to check them out either way.
Edit:
Upon closer inspection they look much smaller. I thought they were on the floor but they’re on a stool.
I was going to say that I believe Roy Allison was still working for AR in 1968...and I dont believe he ever left New England. I live in Natick 2 blocks from where he set up production back in the day...a couple of my friends of my sister worked there and my brother and sister both had Allison 3's at the time....