The old DR201s came with four switching speaker jacks. On some amps the jacks were wired in series/parallel and the OT could be tapped for 4, 8, or 16 ohms.
The schematics I have found on the MHUSS site show the EL34 based DR201s with series/parallel outputs and the KT88 based amps with parallel outputs.
Probably best to confirm the wiring if you get one of these amps. My EL34 based amp has been modified so jacks 1 and 2 are parallel, and jacks 3 and 4 are series. If you only plug into jacks 3 or 4 on this amp you don't get any sound.
The original series/parallel setup was jacks 1 and 2 parallel (this makes up circuit A), and jacks 3 and 4 parallel (this makes up circuit B). You can plug plug into any single jack and the amp works fine. Or, you can plug into both jacks of either circuit and get a parallel connection. Also, if you insert one plug into circuit A and another plug into circuit B, the two circuits are automatically put in series...it doesn't matter which jacks you use in either circuit. Finally, plug into all four jacks and you have series/parallel.
Complicated and confusing, but potentially useful in the old days when bass players might use a really big stack of speakers.
The MHUSS schematic for the DR405 shows the same series/parallel switching scheme.