there's a good chance shimano did everything right when they manufactured your cranks.

it looks to me some previous owner modified your crankset by drilling out the relieved areas in the spider arms..
of course this would substantially weaken it.

unfortunately it's not uncommon to see this modification on campagnolo and their copies of this era.
there is another thread on ss/fg identical to yours about a trashed campy crankset modified in the same way.
i'm pretty sure the only big production crankset of the period that was actually designed and manufactured with drilled arms was the sugino super mighty.

campy record, suntour superbe, first gen. dura-ace & 600, sugino mighty comp, stronglight.. they all had fluted but SOLID arms when they left their factories...

is this what your crankset looked like ?