Stephanie, I think Sandy hit it on the head. The FCC wanted to know where these units would be in the early days. A microwave oven does radiate RF energy which could cause interference to other services. Try locating a WiFi router on channel 6 next to a microwave! It is a bit odd though since the frequency used by microwave ovens falls into the ISM band which were established in 1947 so there wasn't much that RF heating devices could interfere with.
Cory, any idea of the vintage of those cards? Late 70's??