ISMACS may have what you are looking for.
http://www.ismacs.net/singer_sewing_...sses-1-99.html
if you scroll down it has
27-1 treadle
27-2 Hand crank
27-3 treadle - it also says it has a round rear inspection plate, but I don't know if that is a difference or not
27-4 same as 3 but hand crank.
I have found most often, the -# designates the "power" option it came with, so the 27-1 and -2 are the exact same base machine.