The serial number isn't 5100. That's the model, made in the 1930's. Find a local gunsmith in the yellow pages, or search at a gunshow. A 'smith may be able to remove the dent, but either repair or...
Just looked at a friends boito 12 ga...the front wood forearm (attached to the barrels) needs to be pulled off first, like most double barrel's do. Then slide the lever to break open the breach....