Denny, et al,
When I queried both the MILW and MILW Modelers Groups about the car in that ebay photo, Thom Anderson referred me to a Wade Stevenson photo of the car bearing original number MILW 39000, on page 38 in the Morning Sun MILW Color Guide Vol 2, by Doug Nighswonger. The photo caption states:
"One of the strangest cars in The Milwaukee fleet was this lumber car. A one-of-a-kind experiment built from flatcar 650511, it has a 50'9" overall length with a 48' interior length and a 30' door opening. Removable door posts kept the doors in place, which slide one over the other to the left and to the right of the opening. It was built May 2, 1956, and later was renumbered 16800 on August 30, 1962. Shown at Othello, Washington, on October 20, 1961."
This singleton car does fall within the group era, appearing in the Jan 59 ORER as MILW 39000, with data confirming that in the caption above, and with Note DDD: "Car No. 39000 is equipped with wide side doors for loading lumber." It was built on a riveted flatcar, as clearly seen in both photos that have surfaced to date. Unfortunately, the Stevenson photo is too dark to answer one key question for modeling this unique car: what were the ends (and the roof) used in this home build?
HTH, Dave Sieber, Reno NV