I don't know that much about internal geared Sturmey Archer hubs, so you probably should STOP reading now.
SA appears to be a little picky about their indicator chains. I would insert something into the axle without entering into the threads that are deep into the hub ( a small dia. Phillips screwdriver my throw off your measurement). then measure the distance from deep into the hub to the end of your axle. that measurement should correspond closely to one of the indicators listed below. That should match one of the Mark I thru Mark IV part numbers. I would think you could get away with going a little shorter but if the long smooth part of the indicator protrudes past the end of the axle, you might not get the shifting desired.
The other option is to just grab one and see if it works, what the heck...
Please let me know if this works or doesnt work. I have a 3sp internal SA hub that also accepts a 9sp cassette that I am wanting to set up, so I'm looking for a (HSA716)
Sturmey-Archer Indicator Spindle (Chain) :
HSA125 (mark 1 or I) : length 49 mm ;
HSA126 (mark 2 or II) : length 56 mm ;
HSA315 (mark 3 or III) : length 59 mm ;
HSA316 (mark 4 or IIII) : length 62 mm ;
HSA420 (mark 5 or IIIII) : length 66 mm ;
HSA615 (mark 6 or IIIIII) : length 40 mm ;
HSA716 (mark 8 or VIII) : length 75 mm ;
HSA715 (mark 9 or IX) : length 69 mm ;
The length is from the shoulder to the thread that goes deep into the hub to the shoulder where the chain is (on the smooth surface with marking). I'll see if I can find a pic.