[Arran]

It seems to have been a common thing among European radios of the 1930s to insert the speakers into a cloth "bag", But I noticed that not every company did this, and most British ones did not. However whenever I watch a radio restoration video featuring such speakers they are almost always full of as much dust as vacuum cleaner bag. I don't know if the intent was to keep normal household dust out, coal dust, or insects, but they obviously did not do much as they seem to have largely disappeared after the war. I think that the stuff used on those Philco speakers was probably muslin, other then keeping larger debris out it's probably only slightly more effective then bug screen. Other then facing car radio speakers, which are normally installed facing up, I haven't encountered speakers with fabric glued to them, Philco or otherwise.
Regards
Arran