If it is ex Montagu III (1896) then it is lot 640, which was a lot of 2 shillings, with one uniface (possibly rubbed down, though could be genuinely uniface). Sold for £4/10/- but I don't have the buyer. The only other possible ex Montagu is part 5 lot 422, described as 1645 usual type (no final E?), another on a large flan and a third dated 1646 and gilt. Described as well preserved and cost £1/15/-. Bought by Weight, the dealer.
The price of 42/- is close to half the cost of lot 640, so suggests the person who bought it at the sale.
Thinking a bit laterally, RCCB's sale in 1921 had a lot of 4 Newarks including 1645 NEWARKE shilling struck on a large flan showing the original granulated marks on the silver, another with plain reverse (cf. Montagu III lot 640) and a pair of 9ds. None illustrated and no provenances given, but strangely coincidental that he should have a plain reverse. Looking in Eaglen, the N is very similar to those in CB16-3 and C17-1, both in RCCB's hand.
It's a bit tenuous, but sufficient to pursue.