We had pondered when we visited the factory what was the purpose of the little outhouse on which the Royal Enfield Owners Club has placed a plaque. Reader Nigel S. has been there recently and offers a compelling theory: it was the gatehouse for the entrance to the factory. He provides these photos, this one from 1938,
and this one from 2010,
So the picture I had found, apparently dated 1948, that doesn't show the outhouse, must have been taken much earlier.