I had the same problem with a Model 64 bought from the classifieds here on SWF in 2015. The seller was relatively new on SWF and I was not savvy enough to ask him about this. It still has some very noticeable pitting on both sides of the frame under the grips - despite some polishing on my part. Good news is I just saw a tip here yesterday about using an aluminum block as a sanding block to remove it. Even better good news is the fact that, otherwise, it is in super condition despite its age.