I just removed a screw from my Legacy ESE Hudson as the shell screws are listed on the parts breakdown.
I tried it in my 6-11147 M1b and its the same:
SCREW # 6-32 x 5/16" sm
On a side note. My Legacy h10 was delivered missing a tender screw.
There was nothing from any of my other 7 steamers that would fit, nothing available at the local hardware store would fit.
I took a guess and ordered a screw from an online hardware supplier based on the TMCC H10 specs, and it didn't work either.
I ran across a stray screw on my train shelf from god knows what . I think it came extra with a caboose years ago.
Low and behold it fit the H10 tender. Go figure