Hi
I've had a good look through that ED (6) and the previous big one (5), which is in the same hand. I think that it is an S M - S looks like the same form as in shoemaker, etc. Although some of the early ones look a bit L-ish

Also pretty sure it is an M and both letters are similar to the MS (male servant) It's not an L for Labourer as he writes labourer with a small l (lab).
In the previous ED (5)* , there are quite a few neatly written 'seaman'. Could he have got fed up of writing this out and abbreviated it to S M.
Alternatively - maybe sail maker

Gadget
* 5 rather than 5a because 5a is all vessels anyway!