WHBM,
You are correct in guessing Winnipeg for B707 "unspecified" tech stops. My (very old) Junior Jet Club log book entry for 12th December 1973 has me on G-ATZD, LHR-LAX with a flight time of 13 hours and 30 mins. We landed in Winnipeg for fuel. I believe this was a P+W powered aircraft so there were still occasions when loads, winds, weather etc meant the trip couldn't be done in one go even for these later aircraft?