In article , Janet Baraclough.
.. writes

Little girls in Lancashire used to do the same in the fifties. The
indoor-slippers were transported in something called a dorothy-bag with
strings which you could loop on your coat buttons.

At school, we kept our school "indoors" on the premises in a bag hung
from the coat peg. Even in dry summer weather, every playtime involved a
scrum of children laboriously changing their identical "indoors" to
"outdoors" and back again. Most of us probably have deformed feet from
spending large parts of our childhood in two left shoes of different
sizes.

Primary schools still ask for children to wear different shoes indoors -
except now the indoor shoes are plimsolls.
--
Kay
"Do not insult the crocodile until you have crossed the river"