The photos have been marked to have been taken during mid October 1991, at least a month after the Third Series debuted on television.
Seemingly taken after the Third's launch before production of the Fourth, the Thomas and Percy models can be seen along with The Fat Controller and his bodyguards.
Standing and looking beyond the Brendam set is none other than the series' Director from 2004 to 2008, Steve Asquith who helped out with the Classic Series serving in various roles.
At the time of the photos, Asquith took on the role of Production Supervisor before moving to the Models Supervisor position in the Fourth Series.
Also included in the gallery is first narrator Ringo Starr with his Bond girl wife Barbara Bach at a Thomas launch party, the Rev. W. Awdry with the TV Series models and more.
Thanks to SteamTeam and Chris for the alert.