If your serial number ends with an "M", it is a magnum receiver. Some Expresses say "Remington Express Magnum" and others are just "Remington Express" although both can be magnum receivers. The older ones usually just say "Express". I have four 870s (from 20 yrs. old to 5) and none have a shell length stamped on the receiver, just the barrel.

I have always known this to be the way to identify receivers but to remove any doubt, contact Remington's customer support @ www.remington.com. They should be glad to assist.