Okay having the OSSC enough time to understand how Generic mode (best mode IMO) works, so I’ll try to explain.
The multiplier (3x,,3x,4x,5x) in generic mode refers only to the vertical plane while the horizontal plane is adjusted without a multiplier, so if you look at the advanced timing of generic modes you’ll see 960×240 (3x), 1280×240 (4x) and 1536×240 (5x).
V.samplerate which is not show in advanced timing is the 263p (314p, etc..) value you see on the main input display.
From what I observed the monitor/TV will try to choose resolution from its database according to the ACTIVE resolution the OSSC sends.
Generic Line3x mode that sends 960×720 makes my monitor go into 1280×720 mode and will display all content in 4:3 format.
So it appears that the monitor/TV is much less picky about the samplerate (total) pixels, but very sensitive to active pixels.
PC monitors seem to accept practically any active resolution, even 811×643 since they designed to accept all kinds of computers.
Here is a 240p Generic Line2x mode to try that results in a perfect 640×480 4:3 image with no underscan.
It might be more compatible with some TVs than the default Line2x 720×240 with standard NTSC timing.
S.synclen 62 (default)
V.synclen 3 (default)
V.backporch 15 (default)
V.active 240 (default)