Lordy me, this thread goes back a bit. Used to sing this shanty grouped around a stoked up Aga drinking tea and (when it was still politically correct to do so) smoking.
I always understood 'chicken on a raft' to be fried egg on fried bread; a lower mess deck staple but not so tasty after it'd been sitting in a bain marie for too long.
Dabtoes I believe refers to ordinary seamen, whereas Dustmen I believe were stokers. Frd and Aft being where they were individually grouped at mess.