Thermo-

Bellows unit, small             # 070 3310      $3960
Gasket, Gold 11.5 OD x 10 ID    # 055 1240      $ 201

SISweb.com closest match
Gold Wire Ring Vacuum Gasket    # GG040031      $55.53
ID (in)=0.40, ID (mm)=10.2, Diameter (in)=0.031, Diameter (mm)=0.79
10.2mm + 2 * 0.79mm = 11.78mm