Not sure if the on-line International Postage Meter Stamp Catalog is in error, but that document suggests the Mail-o-Mat would be a IA3(k)...either way, it makes for an interesting collectible and the meter number shown on your example certainly fits within the five digit range quoted below:
Quote:
k. Self-service vending machine stamp. 1940, Pitney Bowes "Mailomat", identifiable only by meter numbers 51002-51171. (Not all numbers used.)