It's a crate lid for cooking equipment of some kind.
J2 is the Vocabulary of Army Ordnance Stores section for portable cooking equipment, JA73 is the prefix and first two digits of the stores code.
It's post-1939 equipment, as it's later than anything in this doc:
http://www.essexhmva.co.uk/education...%20Cookers.pdf
So it would be J2/JA.73nn followed by the description.
Chris.