Why do my Beseler enlargers have a spacer between heat absorbing glass and condenser?

[xkaes]

My two genuine Beseler CB7 heat absorbing glasses are 1/8" thick (AKA Double-Strength). To me, the color looks just slightly greener than plain window/framing glass (3/32"). One is factory installed, the other not. Neither has a mark or dot, of any color, alone the edges. Both are slightly smaller in diameter than the condenser tube -- about 1/16" -- which would allow for expansion -- although it is held in place by four metal tabs that would make expansion difficult. (I'll check into that aspect.)

When I get around to it, I'll add it to the top of my condenser assembly while maintaining some space between the two heat absorbing glasses -- there is plenty of room in the condenser structure.