Of course, I am no expert in the manufacture of these lamps. However, the lamp is probably from 1984 rather than 1976. You can see a ‘soft edge’ at the end of the phosphor layer towards the neck and no ‘sharp edge’. In the past, the phosphor was applied ‘wet’ and then mechanically wiped off the neck for further processing. This resulted in a ‘sharp edge’. The later standard production process for coating was ‘dry’, namely electrostatic. This created a ‘soft edge’ at the transition to the neck. Mechanical wiping of the phosphor at the neck was no longer necessary. This is how you can tell the difference.