jw I have five 870s, two 1100s and two 31s. All have machined steel
receivers. All you have to do is field strip any of them and then remove
the trigger group and you can easily see the obvious. Early 31s have a
steel trigger housing but the 1941 version has an alloy trigger housing
like the 870. 870s have more stamped parts than the 31 like the slide
action bars and shell stops but the main difference is that the 31 bolt
is tilted up at the rear by the action bar and locks into a machined
recess in the roof of the receiver while the 870 has a locking lug
that tilts up at the front to engage a recess in the barrel extension.
The 31 is similar to the Winchester mod 12 in that both have chamber
rings or bushings the bolt has to headspace against so both require much
more machining than the 870 so are more expensive to make and so
are "better" in the minds of the owners.