A few year or so ago I was having troubles suggesting that the coil was dodgy but on changing it the problem persisted so further investigations were made. I discovered that the resistance wire was not resisting and was putting out the full voltage produced by the alternator. Worth checking, I fitted a separate ballast resistor as per the earlier models and Toledo's. This works correctly with a ballasted coil. It was also suggested that a normal non ballasted coil would have probably been ok also.
I have had no feed back at all as to the questions asked, do the resistors in the cable fail over time or was it a case of BL changed specs near the end of production and told no one?
The coil you describe looks to be the same as I had fitted and this looked to have come from Rimmers but was fitted before my ownership
The ballast wire on a Sprint is completely contained within the wiring loom, a white lead from the fusebox "in" side is routed via seveal other bits and bobs (Tach, voltmeter etc) to a point in the n/s engine bay near where the washer pump wires emerge (but still within the loom) here it is joined to the ballast lead (pink with white trace) and it immediately doubles back and runs within the loom all the way back across the dash, out through the o/s bulkhead and thence to the coil. The length of the lead gives it the desired resistance. So it is not physically possible for the lead to "lose" resistance (though old age and poor connections MIGHT allow it to gain some) and it still runs at 12v just the resistance is 3 ohms not the normal 1.5. the only real possible failures of the ballast lead are from breaking or shorting, which would result in a total failure to proceed, unless the key was held in the start position (a certain test for ballast resistor failure that mechanics have used for more years than i've been in the trade)
Calling a ballasted coil 6v or 9v is a misnomer, they are all 12v, it's the RESISTANCE that alters. Ask Jeroen - and when he stops cussing in dutch, he'll tell you the same thing!
However, bearing in mind the fact that the cars are over 40 years old and many have lead hard lives, it is more than possible that someone in years gone by has suspected a ballast fault and bypassed the lead with a bit of normal wire, causing a mismatch which will eat coils, points, condensers etc for a hobby! Without chasing all the wires back to source and possibly dissecting the whole loom, it's impossible to tell for sure!
There was NO spec change in the ballast lead for Sprints in the whole 8 year production run.