This is a replacement Deck Cap for Newport Propane Heaters (P9000 and P12000; aka P9 and P12). It fits the Newport proprietary double-walled flue pipe (2-1/2" OD, 1-1/2" ID).
In July 2014, Dickinson made an improvement in the design of the Deck Cap fitting for their Newport Propane Heaters, which made them 3/8" taller.
This resulted in the existing Deck Cap accessories (Cap Guards and Weather Guards) no longer fitting over the new Deck Caps, as they were too short.
Dickinson has designed a new series of these accessories to fit the new, taller, Deck Cap.
These taller Cap Guard and Weather Guard accessories have part numbers ending in "A"
Which Cap Guard do you own?
It's simple:
Measure the distance between the bottom of the two "bells" of your Deck Cap.
If the distance between the bells is 1-5/8", you have the old style - and should use the "non-A" accessories.
If the distance between the two bells is 2", you have the new, taller, version - and should use the "A" accessories.
To avoid the disappointment of accessories not fitting, do not mix the A and non-A parts.