My Besseler 23 has a weird little dark spot in the middle, maybe because of the smaller bulb.
I'm not familiar with that enlarger, but is it a regular frosted bulb with the writing at the end? I had an enlarger where someone had put such a bulb in it - when you turned it on, you could read GE in the middle of the easel.
The lens you have is good for up to 6x9, I think - but definitely for up to 6x7.
This is a great light source for contact printing - it's what I use. I put the filters in where the variable condenser goes.
I usually use a Rodenstock 150mm for 4x5. I think I have a Wollensak somewhere or other. I can't remember who made the 135mm I have. I don't make big prints, so I'm not too picky about the enlarger lens.