Good Morning Alan,
I have just re-read the whole of this thread and now see where the confusion arises. In his original post, Vic mentioned that the short section of exposed coax inner inside the base ..'is itself an antenna of about 19mm', but it's not clear whether he then included this as part of his 69mm total antenna length. If so, the '50mm' vertical part would need to include the vertical section inside the base cone from the point where the coax is soldered on, though that would make the antenna much shorter overall than it looks in Vic's pictures. I suspect therefore that when he says 'cut the top whip to a total of 50mm', he means from the bottom of the screw connector on the left in your photo. This would make the overall length of the vertical part (excluding the horizontal coax, but including the vertical part inside the base) about right. I may however be wrong, so have dropped Vic a PM to ask him to clarify.
As I have said before, this antenna is 'receive only' so its length is far from critical - as long as you don't make it so short that you start to attenuate the received signals as this will have a directly negative effect on the triggering of warnings for 'bearingless' Mode C/S targets.
Watch this space
Regards
Peter