The latch spring or button may be broken/missing, see the FAQs for pics of what should be in there. The notch is mentioned on Fred M's website and has been posted here, I have one on my 35 Rem and 280 Improved barrel, it's a modification to allow a pocket knife tip to remove a stuck case, beats a ramrod or dropping a drill bit down the barrel in the field. The chamber may have a defect in it that causes sticking brass too. A previous owner may have lost one of the parts when they worked on it too, you can order the parts from Brownell's and maybe H&R, I just ordered them from Brownell's recently for the extractor to ejector conversion.
Tim