Here is a link to Teds blog that had some information and a couple of pictures of an M-55a.

http://prototopics.blogspot.com/2019/01/wordy-wednesday-baltimore-ohio-1937-aar.html?m=1

I have a Sunshine M-55 kit I partially built.  The roof doesnt even come close width wise to fit.  Probably a full 1/8 inch to narrow.  I understand I can fill in the gap with styrene but then it doesnt look anything like the car.  So it went up on the shelf of shame and been there ever since.  I will revisit it someday though.  It has the cool sentinel decals.

Scott McDonald