Older, yes.
First generation... I don't think so. I don't know how Remington ran the serial number ranges on 870s, so I can't go from that. But unless the wood has been changed out, it isn't 'right' for a really old gun- they didn't do impressed checkering on the early guns. And the oldest 870 I ever saw had a 'divot' on the right side of the forearm to clear the ejection port, but didn't have a matching one on the left side, and the forearm was ribbed, not checkered.
A very nice 870, glad you appreciate it!
fwiw,
lpl