The original regulator would have been the shorter MCR1 and mounted on two short studs bolted through the rear mudguard just behind the cross tube. Seen fairly clearly in this pic. Ron
Thanks so much for these responses, Ron and Trevor. They're most helpful. From Ron's photo it looks as if the control box is very close to the cross member, which is useful to know. I can eyeball it from there. I like Trevor's idea, too, which might be a good way to go, too.