Hello Frank

Sounds very much like the DuBro stack mufflers (called the DuBro Muff-l-Air) they made in two sizes 09-25 and 29-40 . They work well but the spring coil can clog up with oil and stifle your motor if you use caster .
If you hold it up to the light and can see if its blocked you need to disassemble and clean the spring. If you shorten the spring it lowers back pressure and the stock spring is often too long /restrictive for high performance engines and the instructions detailed this. I have many in use on our fleet.

Regards Gerald
NB added few images pulled off the net but I still have the original instructions if you want me to post.

Hello Frank

I would not worry that some of the muffler has been trimmed off as long as the spring end is still intact , I have trimmed them off short for combat models.
Attached are some copes of the instructions for both the spring  and baffle plate models, also added the instructions from a Tatone Peace Pipe as they are relevant too and a nice muffler if you can find them (my favorite aftermarket 'old' muffler).
Regards Gerald