it has the "2 hump" style receiver. I believe, if the internets didn't lie to me, that the "2 hump" type receiver was phased out to the one hump in 1920?
The one hump version is often referred to as the 520-30, I always figured it was because they started making them that way in 1930 - but I could be wrong. Look at the left side of the receiver in front of the trigger guard and see if it's marked MOD. 520-30 - see the pics of the trench gun at http://www.gunbroker.com/Auction/Vie...Item=331581260
for an example.
I'd get a good 'smith to look at it but as long as it is mechanically sound I'd figure it would handle modern shells just fine.