Hi
wceI think that's great info.!
Your dimensions should help folk down the track.
It's pretty clear to me that the front roller and its single-point
adjuster [as used on the Imperial] was a Morrison design change
to the original Ransomes
Mk 7 Minor ... an improvement.
From what you say, the rollers were 3½" in diameter as spec'd
from new.
Many thanks. 
Please keep us informed as to progress.
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Jackp.s. This is something I have only just noticed.
The very first Victa Imperials did not have single point adjusters.
Front roller height was done in the traditional style - from each side.
The Victa change occurred when Imperials were still 4-strokes.
Best guess, the change occurred in late 1970 for the 1971 season.
This is a new way of dating Imperials.