Hi Alan,
In today's Daily Puzzle, I put in the word 'gallow' which came up as a rare word = fair enough. I put in 'gallows' which came up as unknown. Yes, it is adding a simple 's' to the word but it is a 'gallows', used as a single word where people are hung or other stuff is hung up. I believe there is a case for adding 'gallows' as a common.
Thanks,
Rhino