Pure speculation- it looks like the short section of the under rib was lost and solder was puddled in to replace it but was too high for the forearm to fit. The peening was to move the solder to the edges, in fitting the forearm. The "DF" could have been a former owner, or anyone that had possession of it. I agree with Raimey that it was not Daniela Fanzoj that did it.
Mike