I’d be curious to know if it used to go all the way up as a regular vent.
Never heard of Mission rubber, Fernco goes back to 1964.
http://www.fernco.com.au/company/company-and-history
MISSION RUBBER COMPANY LLC
If we put our name on it… you can depend on it!
For 62 years, Mission Rubber Company has supplied quality, code approved products to the construction industry for the purpose of connecting, adapting, reducing, repairing or capping almost any type of pipe used in sewer and drain waste and vent applications.
1951 for the math challenged 
Well, since I was born in 1951, no wonder I never heard of them. ;):mrgreen:
Seriously, never seen the product around here other than Fernco.
That would for sure be in my report as being sub-standard and being susceptible to frost closure (looks like it’s 2" or 21/2"). This is why it’s a 3 or 4" requirement when exiting via rooftop (varies with climate/area).
This pic is from recent inspection of one that is done right at the rooftop (3") and is nearing “frost closure”. Imagine what a smaller diameter pipe would look like.
heater vent