Just looking at my GE Parts Sheets - I believe that this GE Portable is a Model SP40M1 (could also have a designation of SP40N or SP45N). The part sheets are dated April 1957 - so we can speculate that these portables were built sometime perhaps in 1956 into 1957.
The SP40P MobileMaid that I and other have, have Parts Sheets dated January 1958.
I considered this machine too, since I was just in Cleveland this past Wednesday, but I did not have access to continuous email access during those few days, so I did not try to make contact.
Mike