I think I may have a spare filter drawer in the UK. You don't really need one with the CLS66 head, so as you & Vaughn say tape over it. When I'm next in the UK I'll be moving / dismantling my darkroom and I'm sure I know where it is so if I remember I'll contact you and stick it in the post.
The 601 is a great enlarger, I had 2, the first had very heavy use in a commercial darkroom, I was going to rebuild it and it was in bits, but moved abroad and sent it to landfill, it was quite corroded, and had been rebuilt once before.. But I kept a few bits like the neg carrier, filter drawer, bellows etc which are OK. The others in mothballs.
Ian