The Cumulus Vintage Overdrive is based on the Ibanez® OD-855 Overdrive II, a precursor to the Tube Screamer originally released in 1977. Like other Ibanez pedals, this one was designed by Maxon, whose name is on the original PCB and inside the enclosure, but this one was exclusively released under the Ibanez name.
With its diodes-to-ground clipping setup, the OD-855 actually has a little more in common with an MXR Distortion+ than with a Tube Screamer. It’s notable for being the first appearance of the classic Tube Screamer tone control— although in this case it appears before the clipping stage rather than after, and several values are changed so it doesn’t impact the same frequencies.
The Cumulus is a direct adaptation of the original OD-855, but with the JFET switching removed to convert it to true bypass. A clipping switch has also been added, so you can select between the stock clipping diodes and two other options.
The updated version of the Cumulus is the same as the earlier 1590B version except that the clipping switch has been modified to allow LEDs to be used in the center position as the third diode option.