Various Model T Parts Lists carried Tools with the "Z" prefix, but do not know why Ford picked that designation for his Tools, or when it was first used.
Lincoln
All 3Z and 5Z tools with a three digit number behind them are Model T. 5Z tools with a four digit number behind them are Model A. I beleive the A tool numbers start around 5Z-1732 or somewhere close to that. I would have to look at my book to see what all is listed. I am always looking for the Model A 5Z tools that have a place in my 1929 KRW cabinet. Rod