Why does the 737 Original have an explicit hydraulic-quantity indicator for only the A hydraulic system?
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.