Hello Nick,
Thank you for your comments !
I have to dig up Gebhard Aders, History of German Nightfighting 1917-1945 and some other books on this technical subject to see whether I can find a definitive answer. I did find some info about the early Spanner-sets and it can reasonably assumed that Spanner IV was a further developed variant, but I am always to try to be sure
Wish you a Merry Christmas and - never too soon - Happy New Year !
Marcel