When I installed my 130M , I set it up per the AA instructions using the type M baro, 6" baro tee at the thimble, 5" pipe from the AA with one 90 bend. The notable exception is that I couldn't get the 5" to slip into the 6" tee so I didn't complete it that way. I do not have an excessive natural draft in my 21' chimney.
I did not experience any exhaust gasses coming into the boiler room.
The type M baro was unnecessary except to break the excessive draft on windy days. The intent of the AA factory recommendation is to prevent undesired stoking and overheating the boiler because of excessive draft. Ultimately I set it to the maximum for those occasions. Being a novice I didn't "get that" until after the first year.
The type M baro weight adjustment and pins rusted up quickly due to the acid content of the exhaust. That rendered it useless as a precision instrument, not that it mattered in this case.
The pressure drop, and resulting loss of effluent velocity, created when the gasses pass from the 5" to the 6" pipe caused fly ash to accumulate in the 6" pipe, some 2" deep.
After three seasons my 5" vent pipes needed replacement. Taking lessons from experience and from Yanche and others, I decided to experiment and use 5" all the way from the AA to the vertical clay liner in the chimney. I did this to keep the effluent velocity high for the entire trip from boiler to chimney. I figured that the risk of fly ash accumulating in the 6" would be eliminated. I used a cheaper 5" Fields B34 baro.
The baro, not being required for regulating firing draft, works just fine set to allow relief only during high wind situations.
I do not get gasses in the boiler room when the stoker fires, despite not having the baro tee modified per the AA manual.
There is far less fly ash accumulating in my horizontal vent pipes. It is mostly in the tee snout at the baro.