You are right the mystery crank does look like the Sugino crank. Sugino pioneered the 110/74 format I believe. It is quite possible that shimano were having a change of tack regarding cranks at about that time; IIRC Takagi (who I think had made most good quality shimano branded cranks previously) were still selling their cranks under their own brand but at about that time they were bought out by shimano lock stock and barrel and only made for shimano subsequently. Possibly this move was prompted by (or caused) supply problems... who knows? It is possible (bearing in mind it was only a year between Deore and 'Deer Head') that the launch of the first Deore groupset was delayed for some reason, and crank supply problems might have been it?
Date marks on shimano parts are interesting; parts made by subcontractors (which would have included cranks back then) are often on long lead times and/or made in batches so date marks may be well ahead of sale or release dates . Occasionally I have seen a shimano component where there are three separate date marks; one each (in the metal) for major components (which may have come from a subcontractor) and a third, later one, usually on a sticker, which presumably denotes when the assembly was produced.
The FC-DE** crank series were indeed based on the TA (also Stronglight/Nervar) bolt circle pattern. IIRC the comparative lack of dedicated MTB cranks meant that as late as ~1982/83 the specialized stumpjumper was fitted with TA cyclotouriste cranks; (after that they made their own, or rather sugino made them for spesh).
FWIW there is a nice write-up of the first Deore group here;https://bikeretrogrouch.blogspot.com/2014/07/deore-first-touring-gruppo.html
FWIW around that time I had no strong urge to buy shimano cranks; the dyna drive pedals seemed an obvious blind alley (and they were; you could still buy dyna drive AX cranks, heavily discounted, -complete with threaded adaptors for normal pedals- at least five years after they stopped making them). But you couldn't buy dyna drive pedals, because few managed to ride a full season on them without them wearing out. Later they succeed in putting me off with biopace chainrings. They were the first things to be removed from the 1987 MTB I bought.