H W,
The easiest way of telling the difference between a M1a and a M1b is the pilots. The M1a has the old fashion step on pilot and the M1b has a streamline pilot. Similar to the upgrades the Pennsy did on the K4s.
I don't want to guess what Lionel or MTH were thinking but if it is a M1a model it was how the 6755 engine was built and if it's a M1b its how it looks today at the PRR Museum. Lionel and MTH both built M1a and M1b engines.
JohnB
John, with all due respect the cast pilot does not signify that the locomotive is an M1b. You're correct in that some of the M1a's were rebuilt into M1b's. The spotting difference is the attachment points for the thermal siphons (the siphons themselves are internal to the boiler) which rise in a diagonal line at the rear of the boiler above the firebox. If the siphons are there, it is an M1b. If the sides of the Belpaire are smooth (no siphons) then it is an M1a.
Here are two photos of M1b's, you can see the siphon ends above the running board in front of the cab.
http://www.northeast.railfan.net/images/prr6704.jpg
http://www.northeast.railfan.net/images/prr6792.jpg
M1a locomotives could be found with step pilots:
http://www.northeast.railfan.net/images/prr6758.jpg
with slat pilots:
http://www.northeast.railfan.net/images/prr6759.jpg
or with cast pilots:
http://www.northeast.railfan.net/images/prr6778s.jpg
The lettering on the Lionel tender is the "condensed" style that PRR used in the 1930's.
