Gloucester seem to have missed out 13 in the 1948-49 season too.

In the 1939 programme v Bristol, I note that Gloucester did not field a number 13 shirt, but used 16 instead - although there are other teams which have thought number 13 unlucky and therefore to be avoided, I cannot recall other occasions on which Gloucester have adopted this convention. Bristol of course used letters instead of numbers, as did Leicester - both of them until relatively recent years.