No bulb is a Phillips Photocrescenta PF 605, filter is pale blue not unlike a 82C filter. It not heat glass its tape directly to the top condenser.
My understanding is you're not kitted out for colour printing (correct me if I'm wrong), but maybe the filter was to provide a slightly cooler light to the warmer/reddish lamp? Much the same as you'd use an 82 A-B-C filter to cool down an overly red scene. Another theory is to correct for visual colour deficiency ("colour blindness"), much the same as a few people where blue-tinted glasses, others still wear rose-tinted... there are likely thousands of reasons.
.::Gary Rowan Higgins
One beautiful image is worth
a thousand hours of therapy.
"It is horrifying that we have to fight our own government
to save the environment."