18''' for the size/diameter of the plate.
P for "Plat" or flat caliber.
CHRO for chronograph.
NN for New Negative time setting.
L for Lépine (crown and pendant at top).
15-19 P is for 15-19 Pierres or jewels, number depending on how many jewels.
I suspect this has been re-cased as mentioned by @big case.
Never seen "Anti Magnetique" on an Omega dial, so that's a first for me.
The lack of a crown pusher for the chronograph was confusing to me, however on closer inspection there's a pusher at six.
This got me thinking, so I went to the Omega Bible (AJTT) and there in the chronograph section is a match for your watch.
The case on yours is of much later style and it would originally been cased like the AJTT example.
and the "Anti Magnetique" is no longer a mystery to me.