Vic Elford 68 I think.
Hard to tell the no of buttons (early four vs later five) as functions evolved from earlier or different models:
Attachment 564653
I have this mk V that is surplus to requirements:
Attachment 564637
Regarding “ period correct” date best I can tell is Mk V was new around 64 certainly by 1965 so mkV is more contemporary with 911 than mk IV new which was from around 1961 (I might be wrong on the supercession but pretty likely based on wording in these dated magazine adverts where each mk is described as “new”)
Advert 1961 note the phrasing:
Attachment 564656
Compared to 1965 wording that says the then new mk v seems to be the latest :
Attachment 564657
So while was in market and in use at time I think the mkIV is more aligned to 356 era; the then latest mk v was possibly more correct for well heeled king’s new 1966?
There are certainly a number of speedpilot models and variants dating back to fifties so like car radios some mk are arguably more contemporary especially if fitted new by wealthy owner like a king — although such specialist kit got retained /reused by more frugal rally navigator community.
The real halda king was the late “king cog” a Bristol man called Martin Jubb but there are good navigation equipment specialists like Autochron in the U.K.
Some papers from 1966 might help:
Attachment 564643
Attachment 564644
Some more from 70s
Attachment 564646
Attachment 564655
You will need the correct cogs, ( ideally with natty leather case although a full set is not essential) to calibrate plus cables T connectors, clips etc
Attachment 564649
Attachment 564652
Hope this helps.
Steve