That's right. There is one slot in front of the hole connected to the lamp housing. At the opposite end is the mirror that reflects the light through the condenser, and a slot at an angle just in front of the first slot. Do you have to use heat absorbing glass in front of the filters? I could just get a panel made to cover the slot and use the main slot (in front of the housing for the filters and glass if required).
Slightly OT I have the chance to get a 138S at a very reasonable price and within a few miles of where I live, so it's easy to collect. What should I be looking out for before I buy? e.g. what potentially could be missing that is very hard/expensive to replace.