[time-nuts] 2.5V reference IC in HP E1938 oscillator.

[Rob040 .]

Dear time-nuts,
Can anyone enlighten me by telling which IC has been used for creating the +2.5V reference voltage on the oscillator board of the E1938?It's the only IC in the diagram where the type number is not mentioned. In a post of years back I found LT1121, but that one is a voltage regulator and is used on the control board.
Cheers!