American767 is correct, the -100 formed the basis of the 747SR 'Super Airbus' flown in 1972. This prototype still had the original 3 upper deck windows, but I'm pretty sure that production SR aircraft had the later upper deck window pattern.
The 747SR-100 had the fuel capacity of the standard -100's, but with strengthened fin attachments, centre-body crown splices, undercarriage support, stabiliser root, fin root, wing lower surface, wing/body splice, in-spar ribs, spoilers, ailerons, trailing-edge flap supports, middle/rear spars and leading-edge/nacelle support structures. This was to cope with the SR's short-haul/high-cycle operation.
Capacity was 498, later increased to between 523-528.
Seven of these were built for JAL, starting with line no.221, first flight was in august 1973, deliveries took place over the next two years.
These were powered by de-rated JT9D-7 engines providing 43,500lbs of thrust each.
Later, as American767 pointed out, the -100B mentioned in my previous post was built to SR standard. ANA ordered 17 747SR-100B's powered by 46,500lb CF6-45A engines, first flight was on 11/3/78, (line no.346). JAL took 3 JT9D powered versions too.
It get's more confusing! JAL ordered 2 747-100B(SR/SUD) versions in 1985, these hybrid aircraft had the stretched upper deck of the -300!
The first one flew in febuary 1986. Seating was 25 business and 538 economy.
No -100 freighters were ever built, but the 1973/74 recession put some -100s on to the second-hand market. Desperate for work, Boeing offered freighter conversions to these aircraft at Wichita, Kansas. The hangar there was built for the B-47 bomber of the early 50's, so 747's worked there had to have the tail removed to fit in the hangar!
The conversion work entailed stripping the interior and replacing it with a strengthened cargo floor with cargo handling equipment. A 120-inch by 134-inch cargo door was cut into the left fuselage between the wing trailing-edge and tail.
24 -100's were converted, the first being an ex-AA aircraft for Flying Tigers. Pan-Am and the Imperial Iranian Air Force were also customers.
Additionally, Sabena found it's early -129 aircraft had excess pax capacity, so in 1974 Boeing converted both of them to combis, with the aft deck strenghtened and the cargo door fitted.
As you can see, there is much more to the -100 than just early production 747's.