they are just a random occurence as far as I've seen. my Savage made no4 MK1* is all parts matching and does not have the grooves so even on lend/lease rifles they were hit or miss use.
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
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Bean counters told me I couldn't fire a man for being in a wheelchair, did it anyway. Ramps are expensive.-Cave Johnson.