I bought a used 503CW from KEH. Had been shooting with a 501CM. No manual (it may have answered my question). I notice as I look through the viewfinder that there is a small rectangle in the upper right corner (I'm using a PME45 otherwise it would be opposite obviously). I take off the viewfinder . .nothing. Take out focus screen and there is the tiny clear plastic culprit that obviously is meant to be there. I'm just curious what is it's purpose? I assume it has something to do with TTL for flash?