A lightbulb just went off in my head; I think I know what the tapered rimbase means.

If I understand it, the face of the rimbase, where the trunnion meets it, is put parallel to the slant of the splayed bracket. If that is correct, that would be a little trickier when cutting the master wood pattern for the mold.

On my 1:12, I ran into a problem with not having a rimbase for the tapered carriage, due to putting the trunnions on dead-center of the bore, post-Mueller style, instead of in the old low position as is proper for a 16th century gun; this, and the bulbous, culverin-style breech affected the splay of the carriage, and the spacing at the trunnions. I've had to put sticky tape under the trunions to prevent the gun sliding from side to side whenever the model is handled.

John