The boxes are the same as sotonsteve's and my lamps. My lamps have a date code of 22 which acccording to this date calculator gives Oct 67, 75 or 83. Given that early SLI lamps had a more red than violet colour at turn on, I suspect it is 75 or more likely 83 - unused "late" stock.Heck, I hope they're the violet ones. I might have to go back and exchange them!
Thank you for that link to the date calculator. I've just had a look and was a bit confused by my batch of lamps and their numbers. Three have Thorn labels and one has a Thorn EMI label. Two of the Thorns are stamped 2202 (October 1975 or more likely October 1983 as you suggest, I'm assuming it's the first two numbers you enter into the calculator).
The other Thorn (at the back of the picture) is stamped 0372, so from the calculator you linked to that's March '74 or more likely March '82.
The Thorn EMI has a code of 0722 (July '74 or July '82).
This web site suggests the change in gases which changed the start-up colour from red to voilet happened "shortly after" 1980. I hope the March '82 one was late enough to have the new technology!