The buyer will have to supply a metal backplate and drill again on his backplate.
And aligning the holes precisely is going to be even more difficult than drilling in-situ right from the start.
The target buyer of this item would have a working Model M keyboard already.
He would cut off the rivet heads, separate the layers, and re-assemble all the same parts (cleaned, would be my recommendation) except substituting this barrel plate for his original.
There will be no drilling involved, this plastic barrel plate is drilled, screws installed, and I have verified that it aligns properly on a standard M metal back plate exactly as-is. All the buyer needs to do is put on the nuts (and washers, optional) which are supplied loose.
When you say "drilling in-situ" are you perhaps referring to adding screws, as a repair, only in places where rivet heads are broken off?
This item is intended to facilitate a full dis-assemble and bolt-mod project.