You certainly need draft in a mold to avoid trapping the part. However, on a big mold, small localized features can be levered free and the feature we're discussing is right at the edge of the mold. I missed the halcyon days of dune buggy piracy but with five years laying up parts for glass ocean cruisers, I blew and wedged and beat enuf parts out of and off of molds to know that there's more to successful release than lots of draft.
I can operate the sliding sash of either my front or back window without turning a hair. I would guess that a lack of fixturing at assembly of the hull halves might produce an arrowhead shape with the apex on the centerline. I'm having trouble believing that the Plymouth MN and Sac City IA molds would have rocker in the window mounting flats unless by distortion from tabbing to external cradle or reinforcement. Anyone with an older Burro
who's seen the situation Andy describes?