You continue to post quality original photo's Joe.
I am struggling to keep up with you, but want to point out on the last photo of the very early Green Challenger it shows the "Massey-Harris" lettering on the side of the hood rather than the later and more familiar "Challenger", this is sometimes seen in early advertising. Also interesting to note the earlier drawbar and driver platform plus the magneto is NOT a U4, but a base mount Bosch.
Going back even further and the thread discussion regarding the rear semi mounted mower on the Challenger and your question on the Models No 4 and 5 and the more familar No 6 which here in England was a 706.
I checked all my brochures this weekend and they definately manufactured a No 5 Semi Mounted Mower, so far no mention of a No 4.
Attached copies of the brochure and note the tyre pattern, identical to your photo.
Keep them rolling Joe.
Malcolm