The 5651 is an 87 V (at 3.5 mA) glow discharge voltage reference tube with a miniature B7G 7-pin base. It is very similar to the 0G3/85A2.
The 5651 is used in the power supply of the 128, 502, 581, 585, 549, 535 and other instruments.
Tek part number: 154-0052-00
5651 inside a 549