BevelBitsaBrighton wrote: <snip> For some strange reason it doesn't seem to appear on anyone's fitment lists (can't think why...).
I can

. Apart from the tongue-in-cheek implication in your remark

, it might also be because you're using terminology that didn't exist at the time ... because 'grab rails' didn't exist at the time

!
The nearest contemporary equivalent to what you're after was, of course, a 'carrier' - or to use your other term, a 'rack'.
BevelBitsaBrighton wrote:Anyone know where I might source one? Not looking for something designed to take a top box, just a grab rail, as inconspicuous as possible. Thanks!
For the reason stated above ...
I suspect you'll struggle to find anything 'grab rail-y' off the shelf - and even more, specifically for a Mark 3.
1. Ducati Vintage Store do a rack for the Scrambler. But it's not great looking and in any case I suspect it won't fit on a Mark 3 because of the long(er) seat.
2. At the time, this kind of accessory was normally made and sold by after-market suppliers, not by the motorbike manufacturer - the Ducati Corse/BMW GS/Harley-Davidson accessories business/phenomenon is relatively new. It simply didn't exist when the Mark 3 was made.
In the UK, two of the old time manufacturers were Craven and Stadium. Craven are still in business, but don't list anything for Ducati. But it might be worth contacting them nonetheless. Stadium seems to have disappeared.
3. In Italy, police and other public service bikes were often fitted with racks and panniers (there's a photo of one in Massimo Clarke's book, for example). Nonfango was a top mark, and a year or so ago I sold a period Nonfango carrier and a pair of side cases to Gio in Holland. But where you'd find anything similar now outside a mostro scambio, I really cannot say.
4. If I were you ..
The simplest alternative might well be to bend something up out of some lightweight, e.g. copper, tube, something that suits your needs. Then get it made up in steel and get it chromed.