Flat base, cast engine frame and probably a brass flywheel ... Nice!
SE-2 though isn't it ... as it has the exhaust throttle?
Se1 from looking up the TSB Daniel (that's what I used to ID).
Yep brass flywheel.
If you look on pages 25 & 26 in the book "Mamod ..... The Story of Malins Models" I think you'll find it is an SE2.
The SE1 of that era is a geared engine.
Here's the link to the TSB -
http://www.toysteambible.org.uk/Mamod-SE1.htm?modid=1084&xid=1014