Why does the 737 Original have an explicit hydraulic-quantity indicator for only the A hydraulic system? The 737 (Original, Classic, NG, and MAX) has two primary hydraulic systems (A and B),1 which provide power to the aircraft’s flight controls and other aircraft components (like the wheelbrakes, landing-gear extension/retraction, and nosewheel steering).
On all 737s, the cockpit instrument panels have gauges which provide a readout of the pressure available in both of the primary hydraulic systems.2  Hydraulic quantity, however, is a different matter; on the 737 Original, the cockpit instrumentation features a quantity gauge only for the A hydraulic system, with the B system having to make do with a mere low-quantity warning light (the same type of indicator as with the standby hydraulic system, although on a different one of the instrument panels):

(Diagram originally by the National Transportation Safety Board, page 11 [page 29 of the PDF]; cropped and imageified by me.)
In contrast, the later 737 Classic has quantity gauges for each of the two primary hydraulic systems:

(Diagram originally by the National Transportation Safety Board, page 17 [page 41 of the PDF]; cropped and imageified by me.)
Why doesn’t the Original have quantity gauges for both main hydraulic systems, rather than just for the A system?

1: A third, standby system is unpressurised in normal flight, but can be activated by the flightcrew (or, in a few critical situations, automatically) in the event of a primary hydraulic failure; the 737 also has manual-reversion capability for the ailerons and elevators in the event of a complete hydraulic failure, one of only a few jetliners to still include this capability.
2: The standby hydraulic system, which is normally unpressurised anyways, merely has a warning light that comes on if the pressure within an active standby system falls too low.
Your question: Why doesn’t the Original have quantity gauges for both main hydraulic systems, rather than just for the A system?
On the B737 100/200 airplanes the system B hydraulic reservoir is filled from the system A reservoir.  So, when the system A gauge displays a normal quantity the system B reservoir quantity would also be normal (separate B quantity gauge not required).
If the B hydraulic reservoir &quot;low quantity&quot; warning light illuminated then the A hydraulic reservoir would be at a level of about 1.8 gals (balance line between the A &amp; B reservoirs leaves fluid in the A reservoir if only the B system leaks) and its quantity gauge would indicate so. (and there would be a low quantity or total depletion of the B system reservoir also)
B737-300 airplanes and later models have a differently designed hydraulic system in this regard.