I have seen (and tried myself) such remedies:
- a raised rib, either to raise the point of impact as descibed, possibly for trap shooting, or to compensate for a buttstock that has just not enough drop
- adding weight to adjust balance of the gun, some people just want to have more weight at the barrel end than others. Here probably to compensate the weight of the oversize buttplate that also looks like a recent addition

fuhrmann