That was my first thought as well but when I plug it in I do not see them glow. They do appear to be glass and there is 4 of them but so far they are a complete mystery. I own a TK-140 and until recently owned a TK-250 and from the outside they both resemble this unit but neither of them had these "tubes". I have not been able to find anything on the internet about this piece so it seems it is kind of rare though by no means high end even for its era.