As a former Seattleite I think there is a decent chance this is just another one of restaurateur Ivar Haglund's futuristic adds like the Billboards for submarines [1]. Perhaps a literal acer of clams.

[1] http://seattletimes.com/html/localnews/2009889864_ivar18m.ht...

(the underwater billboards did turn out to be a Hoax/Stunt that were not actually put there in 1954.)