As other Members have stated already, the reason this particular frame design was chosen was to obtain a shorter wheelbase than standard.
Before I started this email I thought back to those days... I had just graduated on to my brother's 19.5" Bates of London..... and can remember that curved seat tubes were available at that time, having been used by both Rensch and Claud Butler on their tandems, so I concluded that Harrison did not have to adopt his design for want of curved tubes.
Neil said that Harrison was the originator of the design, but it was also seen around that time, possibly even earlier on the famous Sun Manxman..Manx TT model..one of the two. some of the Suns that I have seen were made with fairly heavily cast lug, Chater-like ends,mudguard eyes on the fork blades and seat stays, D-section forks and headclips ..all of which indicate early origins.
Years later MKM revived the design for their "Ultimate" model ( there's one in the CR MKM section) The builder of all these Ultimates was a chap called Steve Elsworth, and I recall him telling me that he refused to use any tubing lighter than Columbus SL for the down tube because of the danger of the extra stress at the junction of the seat and down tubes, causing deformation and/or breakage. On occasions with a lighter tubes set, he would either use a thicker gauge tube such as SP, 531ST etc or simply braze a strengthening plate at the junction.
However the Harrison avoids this particular problem to a certain extent because the seat-tube appears to pass in betwen the two oval down tubes. Possibly that joint area is beefed up, in a manner that cannot be seen on the photos.
I was quite impressed by the MKM Ultimate and decided to build a few myself..but in order to avoid ant down-tube breakages ...and also in the spirit of the "aero" age.I built several using Reynolds innovatory "Speedstream" tube sets. The D/T was quite beefy and seemed to be of a thicker than normal gauge...however one frame , a 21" did finally crack at the joint after five or six years of constant use.
The puzzling thing about the Harrison is that even with the dramatic wheel-base shortening design, its wheelbase is still a lengthy 39";the space between the rear tyre and the raked seat tube being big enough to get a fist through. So why go to all that trouble? My Speedstream Ultimates, built for time-trialling on straight out-and-back drag-strip type roads, for 5, 10 and 25 mile races, boasted a rear end measurement of 34 and 3/4", including the minimal "flute in the bracket shell to let the 18mm tub. pass with "fag-paper" clearance. Admittedly they would not have handled very well at all in hilly TTs or Crits!
Norris Lockley
Settle..UK