Interestingly enough Mark, I may have found a few candidates for the private car used by the Thayers. According to Jerry Britton's
Keystone Crossings File Archive, the most commonly-used business car on the Pennsy was the Z74 class. 6 of these cars were converted from the S70 class, 3 of which were delivered prior to 1912.
1) PRR 7505 (first number unknown). Delivered April 1911, named
Maryland and assigned to the General Manager, Eastern Region as of 1929, based out of Philadelphia. Car known to survive at least until 2003, renamed
Martin W. Clement and under restoration in San Antonio.
2) PRR 7506, formerly 7503, neé 37. Delivered 1911, named
Duquesne and assigned to the General Manager as of 1929, based out of Pittsburgh. Renamed
Harrisburg in 1941,
Cape Charles in 1950, back to
Harrisburg in 1952, renumbered to 7506 and renamed
Illinois in 1957. Car known to survive until at least 2003 in Cleveland.
3) PRR 7508, formerly 120 (#4). Delivered 1909, assigned to Vice President-Traffic (I believe this is an error, the Pennsy still was using numerical vice presidents in 1912). As of 1919, assigned to the General Manager, not known where it was based out of. Renumbered to 7508 when 120 (#5) was delivered in 1928, named
Chicagoan in 1929. Renamed
Washington in 1948,
Buffalo in 1956. Car known to survive until at least the early 1990s in Rio Grande, New Jersey.
As an aside, PRR 120 (#5)
Pennsylvania is the car currently owned by Bennett Levin, operating out of the outskirts of Philadelphia.
Source:
Z74 Series Business Car Information for Modelers