Philips Heerlen Tube Production Years/Date Codes?

As I understand it, the production codes for Philips tubes (post 1956) include the factory code, then a single digit for each of the year, month, and later, week code.

So how can you tell if a tube was produced in the 60's or the 70's (or the 80's)? Can anyone tell me when the Philips Heerlen plant stopped producing tubes? The tube in question is a Philips tube branded 'Miniwatt' and looks like later production but I don't know how to nail down the decade.

Thanks, Brian
the tube type will often have a 'variant' number after it, like GA1 GA2, GA3, etc. this would be the upper line above date code. based on that, construction, and lots of experience, the decade can be deduced.