You are correct Anthony.
It is where the divisions of the groups are and that HMC/HSC have set regulations to maintain a standard.
HSC is more historically correct than HMC.
HMC incorporates some parts that are modern but are still in the same theme as what they were in period but are more reliable that the older hard to get original.
The issue is keeping the grids with in the meaning of what they were meant to be and not to devalue that grid.

For example the Breadvan should be in HSS as it is a genuine period All Comer.
The XA Falcon is an HMC vehicle and the Monaro is a HSC vehicle due to its history.
It is the owners right where they race their car but it is the public’s perception of what they see and other perspective car owners that may want to join the groups of which group suits their car.

We must all support each other.

Cheers
Bruce