David II Scotland penny. Reverse "D" privy in 4th qtr. Donatus Mulekyn S:????

very rare with the introduction of a D "privy" on the reverse, been discussed in depth in a BNJ article:

"1949-DISTINGUISHING MARKS ON TH E LATE R ISSUES OF DAVID II By JAMES DAVIDSON, F.S.A. SCOT."

Most likely the mark to signify the mintage from a current moneyer Donatus Mulekyn.

Very rare to see these and through my intermediate search could only find 3 other examples(including groats and 1/2 groats) and no sales for a penny in the last 20 years,

Very interesting example