No chain guard, but I may put it back on now!!!
Wrong. With the chain guard on the chain gets bunched up inside the chain guard case sometimes and that can break the case. The 78's had a thicker case in that area.
With the chain guard off the chain can have a place to escape.
It all depends how close the chain guard is to the chain.
Four things you were very lucky about.
#1 The master link was right there for you.
#2 You did not ask your girl friend to help you push the bike up a hill.
#3 The bike did not go down as a result of the sprocket getting loose.
#4 It was night so not many people saw you broke down.