Naval Jelly works OK to put a gray color to the metal. I've used it quite a lot on engraved pieces over the years.
To keep the color even, it helps to rub the surfaces with a PinkPearl erasure while the stuff is working. Otherwise dark areas can occur and spots in the otherwise plain gray color needed for a French Grey look.
That spotted and discolored look might not be a problem if aging a M/L though.
Other engravers have used things like toilet bowl cleaner and with good results for a French Grey finish.
A darker gray color results from what I've seen.
Many of those products used to have an acid like Hydrochloric in them. Some still do.
Others have gone green and now use citric acid and others as substitutes that may work for the products intended use, but they won't do much to put a gray color on your lock plate.
Printed Circuit Board Etchant will put a gray color onto steel in a hurry.
It's Ferric Chloride by it's chemical name.
You can get a big bottle of it off of Ebay.
Use it in a very small % soln with plain water if you do use it. Always use it at room temp.
It works very fast and no need to heat it or the part(s) to accelerate the process!
2% to 5% soln of Ferric Chloride to Water is plenty strong and it won't take but a few seconds dip in that and quickly back out at room temp to turn the steel gray and start to etch.
The etch is very fine.
Immediately rinse the etched parts off in cold running water after taking them out of the etching soln.
Handle with hooks and/or wooden plugs/handles. Avoid finger prints, oil, grease, etc just like any other metal finishing.
This is the same soln and % I use to etch damascus pattern bbls betw rusting cycles and carding when refinishing.
***You must protect the bore when using this***
I coat the bore(s) with plain shellac. A heavy coat. It stands up to the boiling water cycle of rust bluing needed for Black&White damascus finishing. I recoat the bore(s) at around cycle #7 or #8.
Damascus takes around 15 cycles to complete unlike plain rust bluing or browning that can be done and completed in 4 or 5.