OTK BSF Spindles

Anyone have knowledge or experience with OTK BSF spindles? Never seen em before now, what are they run on? Am familiar with the 1A / 1B kpi spindles, but that’s pretty much it?

Are they interchangeable with the standard spindle?

I think they’re 17mm stubs / 8mm kingpins… probably an option for the 4-cycle or cadet chassis