Martin had choose an interesting solution to build his VF frame.
It's not easy to design a robust frame with an easy system to remove the engine.
We can design a large frame, like an O, so with lot of space to put the engine on with braquets. It's the solution chosen on main frame for 4 cylinders in line. But a strong frame is a frame which is close the engine.
And the shape of a V4 engine will requiere to have a very large frame
A VF engine has 3 fitting holes:
Design a frame means weld tubes between these points and the front handlbar:
And like this, it's not possible to dismout the engine.
This type of frame need to be in two parts.
Honda chose to cut the frameat the bottom, on one side:
This solution present many backdraws, complexe to machine, not easy to assemble, and the frame is not so strong.
The Martin frame is base on an other choice, the rear part of the frame is bolt and the interface is made with to sheets parts.
D is the swingarm fitiing point.
It means that if you remove these two metal sheets, all the rear parts of the bike can be removed (swing arm, wheels, suspension...) and you can remove the engine easely.
Other interesting topics:
Many thanks to the owner of this bike. It will be probably finish end of this year, and I hope we will see it during French old bikes meetings.