Ah I couldn't think! I call it tying wire. Presumably you can buy it from an engineers supplier of via the internet and choose a gauge that fits the holes.
Plenty of it on E bay under 'locking wire'...Try this for example... 360975758515 ...The correct pliers for use with locking wire are on there as well....Ian
I often use locking wire on the end of the brake rod thread. You can still turn and adjust your brake without the nut coming off the end. I bought mine off ebay with the pliers. My Manx even had the oil & petrol pipes conected with locking wire instead of jubilee clips, but that is something I've changed, although it seemed to work without leaks.
['Modern equivalent = Loctite? Or is that too much for the rivet counters?']...
I use Loctite on the nuts that are inside the mudguards including the one that holds the toolbox to the mudguard..Should you lose one of these, all are hard to get at with the wheels in place..
Personally, I never use it on threads into aluminium...I have found a tendency for it to take the thread with it when the bolt is removed at a later date...Ian