Ted, They are both "clones" of a Singer 15.
http://ismacs.net/singer_sewing_machine_company/p15.html
The 30/31 does not have a feed dog drop while the 30 does. There is one other difference I cannot currently recall.
Hope this helps. If not, remember, spool, guide, tension, spring, thread takeup lever, guide, guide at needle, thread needle.