Does anyone have copies they're willing to share or know what archive has the test plant reports for the PRR M1 series of locomotives? I'm most interested in the M1a #6706 test report since it had a Worthington feedwater heater, but would like to obtain all.

The M1 was tested multiple times at Altoona:

1923 M1 #4700 (the first)

1929 M1 #6872

1931 M1a #6706

Thanks,
Joe

The Altoona Model Railroad Club, has a lot of test reports in their club library, which is quite extensive. You might want to make an inquiry there.