Thank you folks!
MsCowpea - I start lettuce seeds in a soiless mix in 4 gal milk containers, and grow then under lights until they reach approximately this size -
These are ready to be transplanted, and I will be moving them into into individual 3" containers this weekend.
When potting up, I use top soil + composted horse manure + pea moss mix (1:1:1); the seedlings stay in pots in a cold frame on the deck until the weather gets warm enough (frost-free nights)... then they go to the cold frames to the garden (if we haven't eaten them by then :-)) You can see the trasplanted lettuce seedlings in the pictures in the 1st post