The B size riser is the bow that Fred developed for his personal use while making the prototype take downs in the years immediately preceding introduction of the TD, 1965 Shotgun latch TD, 1968 I-Beam Walnut TD, etc.
The 60" length of the B riser with #1 limbs is consistent with the length of the Kodiak recurves that he used during the 1950s to mid 1960s as well as the Super Kodiaks used after they were introduced.
The A and C risers were developed after the prototype bows as a part of his system to permit interchanging limbs and risers to put together any length or any weight bow for any use by any bowman.
Decades after its introduction, it is still regarded by many as the easiest and most versatile TD system every developed.