80 Cat Question

[Brian Mole]

I've got a problem with an 80 catalytic converter:-
The problem is, the old cat is empty; nothing in there at all, just a big
tin can. The replacement has a nice big matrix in the can, the old
one not even a coating of bits. Question is, is this a blown out cat
or a half-arsed bypass attempt by previous owner? Is it possible
for the whole matrix to disintegrate and go up the pipe?
Second question (which prompted the cat replacement in the
first place) the car backfires between gears, could this have
ruined the original cat or is it caused by the void where the
cat should be?

TIA ye environmentally sound ones........

Brian