You can still buy Maxima strings, Juststrings.com for one, has them on their website for sale.
I am surprised that you do not have chapter and verse on John's strings, Oliver.
This is what I found out regarding John's string use:
Early ?60s:
Flatwound strings of various manufacturers, including Fender and La Bella.
1966??80s:
Rotosound Swing Bass roundwound strings (.045 - .105) , developed with John, and changed daily.
1989?2001:
Maxima Gold gold-plated handmade strings, changed daily.
This is what he said at the time he switched:
From the August 1989 Guitar Player
The big news is that Rotosounds will not be accompanying the Buzzards on their transcontinental flight. After pioneering the development of roundwound strings with that company in the ?60s and in the process becoming virtually synonymous with the name and the sound, Entwistle has switched to handmade Maxima gold-plated strings. ?They sent me a set to try out, and I really liked them. I found that with the proper action on my basses, I could tune the E string as low as an A and still achieve a true tone. I tried to get Rotosound to explore that direction a year ago and they ignored me, and then came out with a set specially gauged by Billy Sheehan that will tune down to a D! So I got fed up. Billy Sheehan and Mark King can sell Rotosounds; I?m very happy with Maximas.?
And finally, the switchback..
2001?2002:
Rotosound Swing Bass strings, changed daily.
Valentino