</font><font color="blue" class="small">( Mine fell out twice this weekend. The planets must be aligned wrong. Lucky for me I saw it as I made the next pass. Did lose the washer & am running out of places to scavange the bobby pins from /forums/images/graemlins/mad.gif.
No worries regarding the extra holes in the pin. It is way bigger than it needs to be to hold up 1/4 of 400#. )</font>
Three years ago, when I first put into service the 60" MMM on my 21D, the first thing I had happen was one of the mower mount pins fell out - the the right front I believe. Lost the pivot pin, washer and "hair pin" retainer.
Went down to my NH dealer, bought a new pin and washer, along with two standard cotter pins. I replaced the front pivot hair pins with standard cotter pins (left the original hair pins in the rear pivots) and haven't lost one since.
In my case, I believe the hair pin was getting snagged by brush or something similar and was simply pulled out. Eventually the pin vibrated out and was lost. I'm always off in the choppin's with my tractor so it'd be easy to see how this could happen in my case.