As John has mentioned, I think you are referring to the body number plate. AFAIK it was used on Speke & Canley built cars, but not Solihull (where BL were using the VIN system & had the VIN stamped into the boot rain gutter channel as well).
The location of the plate is usually on the left side of the bonnet hinge panel, although some diagrams seem to show it located more to the centre of the hinge panel (like Moss do).
Some cars seem to have 2 plates, the only explanation I have heard for this is that one refers to the bare shell, & the second refers to the trimmed shell before it went for mechanical assembly on the line. No one seems to know for sure!
I have never seen any explanation of what the numbers actually mean.
Tony

<font size="1">1981 2L FHC Cavalry Blue
1980 3.5L V8 DHC Jaguar Regency Red - sadly sold!
1977 TR8 FHC EFI Factory development car Inca Yellow</font id="size1">