5513 Meter's first lume glow vs no glow

[statuskuo]

Does anyone know if the lume on 5513 meter's first sub's (1.9m s/n range) should be glowing? I have seen examples that glow brightly then fade quickly, but I have also seen several examples that have little to zero glow, only showing very small specks of luminescence.

I know Rolex was experimenting with different mixes of tritium / zinc sulfide in the 60's so perhaps there are versions that are simply different. My understanding is that later 5513's typically do not glow at all anymore (late 60-70's).