[Graham]

I hope you're only joking Graham...

Not 'modified' but standard chassis as supplied by Scalextric, doesn't come with the plastic hatch. 



The brass ballast is glued in the hole, it's not a working part of the chassis.

So before I go any further this obviously needs 100% clearing up please...is this Class A or not?

Rule reference:
General
G5) Ballast may be added but must be inside or as part of the chassis.

Had a think about this "loophole" in the rules, obviously you have interpreted the rules differently than what they were intended.
Basically the "standard chassis" should be as the RTR car chassis assembly was supplied, accepting your point that actually it is a combination of a chassis section with a chip access hatch, I would still see this as a modification of the chassis. If you take the chassis argument with the chip hatch hole,  then you have filled the hole with a fixing (brass piece) which would not have been included on the original model.

So Basically by replacing one part of the chassis assembly with another it is a modification of the standard model and would be classified as a Class B construction.