We came across this 157-067 "tube" at the museum.  A 157- prefix is reserved for Electron Tubes (Matched, selected, and sets from 154s).  We had never seen one before and since it didn't have a mate it must have been selected.

We decided to have a look inside.  Inside was a tube and two resistors in sleeving.

The tube is an Amperex OG3 voltage regulator.  The resistors are 300K and 106K.

We were still curious as to why this tube was made this way.  We looked up 157-0067-00 in the Replaceable Parts Registry and learned it was only used in the Type 661 Oscilloscope.  The 661 was a sampling oscilloscope introduced in 1961 with two plug-in slots.  Seven plug-ins were available for the 661 - four sampling plug-ins ranging from 0.1 to 0.35 nS and three timing plug-ins.

A look in the 661 parts lists shows this tube as V719, a checked OG3.

A look at the schematic shows this as the regulator tube for the +300V supply.  The metal shield and the two resistors are clearly shown on the schematic.

For further information we consulted the circuit description in the instrument manual.

LOW VOLTAGE POWER SUPPLIES
General
The Type 661 Oscilloscope regulated power supplies are designed for exceptional short and long term stability. Sampling circuitry demands current pulses from the -100, +100- and +300 volt supplies at irregular intervals. Thus, power supply leads and ground return current paths are carefully planned. All non-constant loads are supplied via individual leads from a common point at the output of the regulator circuits, and most grounds are returned to a common point at each regulator. The +300 volt supply reference tube V719 is temperature isolated for short term stability. The CRT display jitter is maintained at a very low level by use of high-gain feedback amplifiers within each regulator circuit.

So, thanks to the in-depth manuals that Tektronix produced, the mystery of this part is solved.  We also suspect that the shield both for temperature stability and to keep the tube in the dark.