[LMH]

Talbot makes (or made?) two bases that can be used on the 300 JR, standard base and elongated base. I think (?) the standard base is what I've seen when used on most original Cobras. The Walpres mirror is slightly different. Slightly shorter in length and the mirror slightly larger though I haven't looked at two side by side.
Could what you're seeing be the mirror in different adjustment positions?
I know what you mean about NOS mirrors and their price. They are like gold!
Larry