The Guild SB-502E is, in may ways, an active version of the Guild SB-202. It has two 'special design' pickups, and extra controls, but in most other ways is identical, having the same body style, construction, inlays, and hardware. One difference is that the pickups and controls are body, rather than scratchplate, mounted. Naturally the bass is back-routed, so unlike the other Guild basses of this time, the SB-502E controls are accessed by removing the cover on the rear of the bass.
The Guild SB-502E was only produced in small numbers and are not widely seen in the vintage guitar market today.
The following text is taken from the 1982 Guild catalog
For the more electronic oriented player who requires an extremely wide range of tonal potentials, we created our SB-502E model. Features active, on board equialization, two Guild special design pickups, two volume controls, bass EQ, treble EQ, pickup selector & phase switch, pre-amp on/off switch. Complete instructions, schematics and recommended setting are provided
The SB-502E was an active bass guitar, with a battery powering the internal circuitry