The Tektronix TM503 is a 3-bay mainframe for the TM500 system. All bays are low-power.
The TM503A has a swivelling carrying handle.
The TM503B also allows the power supply/backplane part to be removed from the case so that modules to be repaired on the bench without using extenders (see link below). It also has openings for the TM5000 GPIB feature, but not the connectors nor the power good signal for TM5000 plugins.
Option 2 adds internal pin headers for each slot as well as rear 50-pin and BNC connectors, allowing for custom wiring to, from and between plugins.
On the standard TM503, cutouts for three BNC sockets have been made in the rear frame, meaning that the user only needs to drill holes in the rear cover plate to install the sockets.