Russian Decapod 2-10-0 Mystery Fitting

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On Bachmann's Spectrum HO Russian Decapod there is a short right angle pipe fitting sticking out of a circular base on the left hand side of the race behind the steam dome. What is the function of this fitting on the prototype?
modelbob said:
Got a photo handy? Without the model, I'm at a bit of a loss here.
Bob See attached photo from page 61 of MR/Linn Wescott's Cyclopedia Vol. 1 - Steam Locomotives. The mystery fitting appears to be a valve handle sticking out of an opening or recess in a shallow dome directly above the air pump and just forward of the fire box.

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