Jack good question there.There is a few changes to the bases,but they are very minimal.
One of which the casting on the Mark II is a lot better than on the Mark I.Like around the the Victa logo on the Mark II there are no cast lines also around the handle for the rear flap as well as a few other places(more listed below).The Mark I's had these cast marks.Also I'm not sure if this is how they were,but from my observation the Mark I's had a (I) cast into the base where the engine mounts.Marked below in red.Where as the Mark II's (From what I have seen so far) had a (2 or 3) cast into them.
The blue lines are the cast marks I was talking about.
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The only other change is the Mark II had extra holes put into the side for the new style height adjuster which was the plastic type.
Mark I
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Mark II
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Other than that there really isn't anything else I can see or have found that is different.
I haven't taken any pictures of the Mark II's crankcase but it is the same as the Mark I's number except as the model number is different it reads 76720 then the serial number.

Here for a good time,not a long time.