An excellent question! The last batch of X29s built, post 1930, were built with experimental AB braked with a transverse mounted reservoir. When these were upgraded to the more conventional AB brakes, by and large they retained the transverse mounting. However, when the pre-1930 production cars were upgraded to AB brakes from KD, they received longitudinally mounted reservoirs. So, in reality, the longitudinal reservoir marks the car as NOT post 1930 construction (so either the 1st or 2nd batch, not the 3rd), but in combination with the other details confirms that the car is from the 1st batch.
From: main@RealSTMFC.groups.io <main@RealSTMFC.groups.io> on behalf of Guy Wilber via groups.io <guycwilber@...>
Sent: Saturday, July 16, 2022 8:33 AM
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Subject: [EXT] Re: [RealSTMFC] Photo: PRR Boxcar 570062 (1941)
Bruce Smith wrote:
“Notable features include the short upper door track, 1/3 and 2/3 height door stops, car builder’s doors, plate ends, Carmer uncoupling devices, longitudinal AB reservoir
These all mark it as from the 1st production batch circa 1926."
How does a longitudinally mounted "AB” brake reservoir help mark a car built in 1926 which would not have been built with an “AB” brake system?