Single edged razor blades were the standard in kits. The shift now is to scalpels Because they are a better quality blace overall. Scalpels also come in a number of blade shapes giving you a choice of edge profile rather than the straight flat razor blade.

Also scalpels have a stub where the handle is attached in normal use. The stub is what you hold until you make a handle. Along with being better quality steel the overall blade is stronger than a razor blade.

Another choice would be exacto blades. Same thing you hace a choice of shapes. But they don't have as good as quality of blade or edge as a scalpel.

You can also choose the blade of a utility knife. But you are limited to only one style blade.

Because of the size of PSK's you are limited to a blade without a handle. Not enough room in the kit. Even with a razor blade to you need to to improvise a handle to do more than very fine cuts. But a stick and some judicious lashing can make a fairly respectable knife. At least for light weight jobs.