/The 500 series Avenger is a "slide thru" model. Rather than a wing saddle you build up the fuse, then cut out the portion of the formers shown on the plan...
This is a 40 inch blow up of the 500 series plan:

When I blow these things up, I use thicker balsa than you'll find in the 500 kit, so I cut the formers BEFORE assembly. When you build the 16" plane, you can cut em after you put it together. I use little surgical scissors...
When we build em really big, (this is the 500 series F6F plan blown to 10 foot...) We build the belly as a separate part, (foreground...)
http://www.parkerinfo.com/ap108.htm