Look at the other side...it should be a little steel wedge soldered in place, not straight solder.
If the former, a gunsmith can make you a new wedge and solder it in.
If the latter, you could probably fill the cavity with some lead solder yourself, but make sure it's not acid core and don't overdo the heat. Just enough to fill it proud and then file it flush. Use rosin for flux.
If the rib is loose (and I suspect it is), you will need to get a smith to fix it.
If it's just the solder, it's really cosmetic...if you hunt in the rain you probably don't want water getting in there so it's a good idea to plug it. Frankly, you could probably just fill it with silicone and leave it be.
