Officially, NSR intended these to be M2 (i.e. 2mm diameter) holes, hence the useless 2mmL x 2mmD (b/c they are Too Short) pack of M2s that should be included in your jewel case.
IMHO – this decision makes no sense from NSR (e.g. does not really make the chassis “uglier”, or lose performance, to make the uprights fat enough for their typical 4-40 screws).
My first black/medium chassis accepted the M2s and I have SCC 4mm Oval-heads on the Top and Bottom.
PER MY PREVIOUS POST #16963, on my Green/X-Hard chassis the top axle adjustment screw holes *were* tight enough for M2s, yet the Bottoms were loose, forcing me to use 4-40 brass grubs (very carefully).
NOTES My new LEB Hobbies car, has wide bottom holes (needs m2.5 or 4-40). Yet – I have another spare black chassis that Is Tight Enough for the M2s (WTH?)
For the bottom – if you have M2.5 grubs (from Thunderslot?) they *may* be the perfect solution.
Otherwise you may want to TRY USING 4-40s (i.e. normal NSR, i.e. use a 0.50 wrench).
The English bit size is: 7/64 (i.e. 0.1094) which is just small enough that the 4-40 (i.e. 0.1120) can Bite into the plastic and hold.
Using the 4-40, you will see the stretch-marks, but unless you are ham-fisted the risk of damaging the chassis is Moderate. Take it slow and careful, one side at a time.
"... get on your bad motor scooter and ride!"
Sammy with Montrose