Atlas 250W MBF/U Kolorlux Mercury vapour lamps

2 of a lot of 5 of these I managed to bag the other day!

and while unboxing and checking them over I noticed 1 was a bit different then the others, I expected them all to be the same!

most where like the one on the right, with the "button" E40 cap and the feathered edge to the Phosphor line

but one, the one on the left has a much more "modern" looking cap, but a straight edge to the phosphor line

curiously the one with the more modern looking cap dates older November 1970 or 1975, where as the one with the older looking cap dates to October 1971 or 1976

I find this quite interesting and it would be interesting if anyone has any input on it, perhaps the Modern looking cap was actually introduced much earlier then I think it was?

I also find it interesting as it helps narrow down any potential production changes, to help further date lamps on short cycle date codes

All lamps came with little paper instructions dated to 1968 for what thats worth