The Tektronix Type 221 is a miniature 5 MHz analog scope introduced in 1973.
The 221 has a switching power supply and built-in rechargeable batteries (which are a maintenance issue). It has a transformer-coupled trigger circuit which would seem to preclude DC level-based triggering. The trigger circuit uses a 1N3713 1 mA tunnel diode. The vertical output amplifier that drives the CRT plates is two opamps made from discrete transistors. One drives the top deflection plate and the other drives the bottom plate.
The power supply in 221 uses the 120-0876-00 transformer.
MOD 290A is found on a version made for IBM, which adds a Single Sweep function, and a "Property of IBM" engraving.