Garth Groff wrote:
"The mystery deepens. My October 1958 ORER lists 231 cars in series 8000-8299. Likely the number series was reused for other cars. Maybe. These cars had a 2081 capacity and were rated for 100,000 lbs. Principal dimensions were a 33' inside length, 10' 4" inside width; 34' overall length; 10' 5" height to top of sides; 11' 7" extreme height which could indicate a vertical brake staff or a heap shield. Does this match dimensions you have?"
No. M 8000-series dimensions are as David stated, and are characteristic of PRR Class GLA. According to Gene P. Shaeffer's "The Montour Railroad, M 8000-8099 are ex-C&O cars from C&O 40000-43749, obtained in trade for 111 Montour gondolas in 1937. (Montour January 1945 ORER entry attached.)
The number series was reused. M 8000-8299 were rebuilt in 1957 from M 18000-18299, AAR alternate standard offset twins built by Pullman-Standard in 1941.