Was mutton marketed as lamb back then? The local butcher always said it was lamb.
All I know is that my mother, would never buy a leg more than 3 lbs. and said "mutton" is the sh!t those English people eat.
Soak mutton (sheep) in pineapple juice and salt for a week. Let it dry hanging for two days. Cut out any spec of fat. The fat even tastes like waxy rancid nastiness right from slaughter. If you do that, mutton (sheep) is ok. Any fatality or accident should not go to waste. Always a few every year. Mostly from tipped over ewes.
My parents would alternatively call it lamb or mutton. I don’t know if they made a distinction or it was based on what they found in the store.
Mutton is full grown sheep. Lamb is a tender yummy young sheep.