The retainless design means that they do not drill completely through the body of the valve to enable installation of the hinge-pin and stop-pin of a dual-plate check. Instead of drilling through, they machine out a recess. This is pretty standard design (retainerless) by most manufacturers.
See API-594 for some pics of these pins through the body and google retainerless for pics of the variation.
Sorry about leading you astray as I was equating the retainerless description to an ordering description we used on ball checks. A better comparison probably would have been caged or non-caged.
The phrase Fugitive Emissions was just becoming the buzz word and you couldn't run fast enough to give anyone a dual wafer check valve at our site, though we did use a single disk internal hinged check valve.
thanks for your explanation. I have found more info about "retainerless" on web. On API 594 valves droawings are all with the retainers passing through the valve body.