Hi Louie,
Many thanks for taking the time to reply. My filter drawer also has a lip on all four sides - not surprisingly I guess, but I remember my ancient Meopta, (also purchased second-hand); having an alloy four-sided retaining clip in addition to an opaque glass. I hadn't realised that the glass was itself a heat absorbing filter. Incidentally, I've since noticed that the drawer to each enlarger are different sizes so doubtless, I'm going to have a mother of a time tracking down the glass inserts. Thanks again.
Richard.