Since maybe the 1950s lab grade time and frequency equipment has used 24 volts DC as a power supply.  The HP 5087A 24 Volt Backup power supply was used to power the 5060A flying clock Cesium standards.  The HP 100 series of instruments all use 24 VDC power sources.

This particular unit came from eBay with a foam filled bag in the battery space, like it has never been used.

Battery Compartment Size

left to right 13" max
front to back 7.25"
height 5" max

Front Panel

Meter switch Output or Battery :0 - 40 Volt meter, -5, 0, +5 Amp meter
Fuse Failure lamps: AC, AC, OUT, Battery
Battery Charge lamp & Switch
Interrupt lamp & reset switch
AC Power Lamp

Rear Panel

Ground Post
J1 - AC power connector 3 pins  (cable connector = 97-3106A-10SL-3S)

• Black (hot) = pin A
• White (neutral) = pin B
• Ground = pin C

The power cable might be called a CX-3277

F1, F2 - Fuses
J2 - Alarm output 6 pin MS _______________
J3 - External Battery  4 sockets (cable connector = 97-3106B14S-2P + M85049/41-6A clamp)
J4 - Clock Out 4 sockets  (cable connector = 97-3106A-10SL-3S) (this is only a 24 volt output)
J5 - Oscillator Out 5 sockets 97-_____?? (this is only a 24 volt output)
J6, J7, J8, J9 - 24 Volt Outputs 4 Sockets (cable connector = 97-3106B14S-2P + M85049/41-6A clamp)
For those applications where a MS grade connector is not needed I'll be using Power Pole connectors in a 24 volt configuration.

Power Supply near Transformer

This PCB has a single multiturn pot and a fuse.

Power Supply with + Violet wire to battery

The PCB has two multiturn pots and a fuse.

Battery Compartment

The battery compartment has a clamping mechanism that will take up to about 13.5 x 7.25" of batteries.  This would hold 4 each 12 Volt 18 AH Gel cells like the TY-12-18.  The quick connect connectors are about 0.266" type, not the more common 0.187" type like on the batteries, so if two batteries are used they need to be more like 40 Ah types.  The wire colors are +Violet and -Red (seems strange).

I'm using a couple of the very common 7AH 12 Volt Sealed Lead Acid (SLA) batteries instead of the much larger batteries that the 1290 was designed to hold.  My concern is not long term power outage so much as outages that last for a few seconds.  These batteries are not near as big so I'm looking at using a 1 foot long 2x4 as a rear spacer so that the battery clamp can operate. 

The quick fit terminals will need to be changed to the smaller 0.187" type and a battery to battery jumper wire made up.

Pots

Looking from front of unit Ra to R3 left to right:
The thing draws 21 VA when idle no batteries. Mostly due to the xformer.
Info thanks to Paul WB8TSL

Austron Time & Frequency Instruments (24 Volt standby power)
FTS 4060 Cesium Standard (24 volt standby power)
HP 5060A Cesium Standard (24 volt standby power)

William Perry has the connectors for a reasonable price.