Cam swap ,i guess im living right

Tore my 408 down today for the switch from a hyd to a solid flat tappit cam.When i pulled the roller rocker shaft assemblies i noticed a retainer on the right side had a funny angle to it.Upon a quick glance i noticed it was missing a keeper .I have spun this motor to 6000 many times on the track (150 passes),and i just did it again the other day in my driveway before i tore it down.How it kept from popping the other keeper out is beyond me.The whole thing woulda been done in a micro second if it had droped the valve at 6000 rpms.I cant find the keeper (i fished around the pan sump with a long magnet and still nothing),so does anyone know how i can tell which one it is without pulling the head ? I can replace it easy enough with a spring compressor .Will i have to get a whole set for the one i need.Please excuse my ignorance as im still fairly new to Mopars ,but not engines.These are econo W2 heads BTW.The valve stem tips (including this one ) look text book perfect ,as do the retainers .I guess i floated the spring at one time and caused it to pop out.No telling how long ago.Thanks for your help ...