word puzzle question

[Jean-Joseph Cote]

Now that I think of it, when this appeared as teh Puzzlah on Car Talk, I
think they had the additional restriction that adding the 's' changed the
word from feminine to masculine, as well as changing it from plural to
singular.  Given those rules, you can maybe also get away with 

abbés -> abbess

although there's the pesky detail of that accent, and I don't know
whether "abbé" fully qualifies as an English work.  But working from the
pairs that Pierre posted, I also found

deadlines -> deadliness

And giving it more thought while I was doing too much driving yesterday,
I came up with the very nice

bras -> brass

I'd like to find one where the original plural noun doesn't end with 's'.

Jean-Joseph