According to early WBCCI Membership Directories:
1958-64: #1366 - Loyd & Marguerite Larson, Muskegon, MI, no caravans.
1978-91: #6244 - Loyd R & Marguerite Larson, Muskegon, MI, caravan 114 - Alaska Summer 1977
Obviously the Larson's rejoined the club in 1977 (being first relisted in the 1978 Directory), probably to join the 1977 Alaska caravan, but their original WBCCI number was apparently unavailable at that time.
Between
1963 and 1977, #6244 was assigned to other members in the states of Washington and Ohio, neither of which I believe owned the Airstream in question. I did not check #6244 after 1995 (it was unassigned from 1992-95). It has likely been reassigned since 1995.