Yep, I've never seen those groove cover nuts on a M5 S2. I'm pretty sure the spring clip had been introduced by that point and am sure I've seen an ad to confirm it. That type, from what I've read, were designed to hold the fabled edger attachment to the deck. It would be interesting to find out exactly when the spring clip was adopted, as it would indicate exactly when the plans for the original edger attachment were finally abandoned - i'll see what I can dig up.
There is a slight possibility that they were added on request by a dealer, as it has been claimed that many dealers did add options to mowers much like car dealers do today. You supposedly could ask for specific components such as chokes, snorkle lengths etc, but i reckon in this instance it was probably a later addition by a hopeful re-seller
At best, I reckon that mower is probably worth 100-120,but I guess there's one born every day, so who knows
