This is an antique / vintage military or livery button with a monogram on the front made from the initials LR or RL.
Looks like there's a tiny number 1 underneath the Kings crown, not sure how relevant this is.
Maker's mark on the back is Jennens & Co., London.
I should know what this button is but I cannot think of it.
Unidentified.
Please see close-up photographs provided for more details and visual confirmation of condition for yourself.
Condition report:
Normal signs of age and use wear.
It measures approx. 26mm in diameter.
My stock ref : ESK/15823 in their own badges box.