I have a question regarding Check valves, what means for a check valve the term "retainer less design"?

Is this in reference to a ball check?

If so this mean that the ball isn't restrained in its movement, normally lift. 

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.

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