Cylinder boring without a cylinder borer.

[Toolmaker51]

The mystery port now appears to be a DOHC chain-well, seeing side view of the cylinder. In pic 04, the supposed 'port' is peeking out above the torque plate, mounted to fixture proper.
Aermacchi's are triple uncommon in the US; though they supported the small motorcycle ventures of Harley Davidson [under AMF] awhile, in 100 and 350cc. That laydown cylinder is more than distinctive.
Thank's for photo's and clarification.