The PRR B74b roof, vent, side doors and end doors are now available at Keystone Details! The kits can be ordered as single sets or two pack sets. Instructions for obtaining ther etchings are on the shapeways site.
Here is a pic of the completed model, note that paint and decals are not included:

The single pack will have the etched parts shipped separately and the Roof and vents from Shapeways. the Etched parts are included in the Shapeways price. It can be ordered here:
https://www.shapeways.com/product/FCPMBFQA6/n-scale-prr-b74b-roof-and-door-kit-for-mt-horse-ca?optionId=60051215Included in the single kit will be:
1X End door fret:

1X four small two window side door fret:

1X two large 4 window side door fret:

1X roof/vent combo model:

The two pack will have the etched parts shipped separately and the Roof and vents from Shapeways. the Etched parts are included in the Shapeways price. It can be ordered here:
https://www.shapeways.com/product/X8A3X5H6D/n-scale-prr-b74b-roof-and-door-kit-2pk-for-mt-hors?optionId=60051261Included in the single kit will be:
2X End door fret:

2X four small two window side door fret:

2X two large 4 window side door fret:

2X roof/vent combo model:

Instructions to build the care are forthcoming and will be available on the shapeways site.