If the Willard is the famous one, the 6309-7040 is the important one. Twelve years in production from roughly 1976 to 1988, sold worldwide in enormous numbers, and shaped like nothing from Switzerland: the cushion case that gave the Turtle its name. It is the most commercially significant vintage Seiko diver, the reference through which most collectors enter the vintage side of the catalog.
The long run left a rich variant map: the first series with square crown guards, later production alongside the slim-case -729A sibling, the Japan-only 6306 twin with its kanji day wheel, and a start date the records split on, 1976 in most accounts, early 1977 in crowd-sourced caseback data. Abundance cuts both ways: parts are everywhere, service is easy, and aftermarket dials, hands and inserts outnumber originals in the wild by a wide margin.
The people's vintage diver. There are enough honest Turtles that nobody needs to buy a doubtful one, and that is the discipline: original dial code matching the serial's production window, era-correct insert, factory hands. A watch that fails any of those is a wearer, priced accordingly, and there is a market for those too as long as everyone is honest about it.
Production windows and specifications compiled from documented collector references and period catalogs, cross-checked across independent sources; manufacturer records for this era are approximate by Seiko's own account.