I don't know of any lens from a 6x6 that will come close to covering 6x12. Very few 6x9 lenses will; most cameras with a fixed lens had a lens that was the least expensive that would cover the format with the requisite image quality. The original lens from that camera was probably a 135 mm, likely a Rapid Rectilinear if f/7.7 or slower, possibly a triplet or Tessar type in later models. The lenses from 9x12 plate cameras are excellent candidates for this; the Tessar types should cover 6x12 nicely (they'll just cover 4x5) and as long as it's unit focusing you're all good (the scale will likely even match). None of my four plate camera lenses (two Tessars, a Skopar, and a Radionar, all 13.5 cm f/4.5) is as much as 40 mm deep; all are made to fold into cameras that are only about 40 mm thick when folded.