Beaver barf doesn't use it. Its to prevent damage to the softer hardwood handguard, the barf resin is hard enough to not need it.
you dont need the spacer if it doesn't fit in place.
there is many different styles for vz58 parts. new and old. i believe the spacer is for the old design hand guard. it all depends on the parts you have.
this covers some of the changes in designs. unfortunately most of the pictures are not showing anymore.
h ttp://www.theakforum.net/forums/66-czechoslovakian/136462-vz-58-part-variations-reference.html
The two 858s I owned both didn't have it.