As pointed out, the differences would basically be what was done to the rifle to make it legal after that ban.
The "pre-ban" rifles will retain all the normal features intact:
* bayonet lug or underfolding spike bayonet assembly
* threaded muzzle
* military style buttstock and pistol grip
* straight-cut on the back end of the receiver
The "post-ban" rifles (MAK-90's) will have those normal features removed or neutured:
* bayonet lug or underfolding spike bayonet assembly cut-off or filed down
* threads on muzzle turned down to smooth or a muzzle cap tack-welded on
* "thumbhole" style buttstock with no individual pistol grip
* often times the straight-cut on the back end of the receiver cut on a slanted angle to prevent the military style buttstock from a solid fit.