You're probably right. What happened, I believe, is that the chain rubbed the chain cover, (I saw evidence on the outside of the chain) and caught the keeper pictured above. The odd thing is that the keeper itself resurfaced in my garage, while the chain flew out on to the road. Like I say, odd, but I've seen stranger in my line of business, I guess. I ordered a new chain yesterday, and I suppose a new chain cover is in order, too. The cover is turning into a real POS, needing a spot weld from before, and now cracked at the other end by the torque of the chain flying off.
Two weeks ago I was pricing a basket case to make a long term project, and I guess she got jealous, coz ever since I've been chasing repairs on her...