Sounds like one I purchased in Miami (keys) back in the late 80s..
Had a semi triangular attachment on top that had a rubber diaphragm inside to make it one-way, so the water didn’t come into the snorkel.
The main feature two channels in the uptube
Exhale went up one channel and inhale came in the other.
Co2 is heavier , so when you exhale in a regular snorkel there is still an amount of co2 exhale in the uptube and the next inhale is partially exhale and not 100 % fresh air.
The 2 channel uptube guarantees every inhale is pure fresh air .
Simple but effective /-).