These are obsolete. I guess you could make your own from an neon and an LDR. You may be able to remove the cover off the ones you have and replace the parts.
I found a data sheet for the VTL-9B8 VTL9A.pdf
The other number 21L-255 I don't recognize.
I would not do surgery on these parts until you are 100% sure they are the problem. Have you searched for a schematic or asked Besler for one?