1471, 1475, 2140, 2144 - only adjust bobbin tension. In Pfaff club, they used to push getting an extra bobbin case. We did about anything possible with those machines, including ribbon embroidery where the ribbon is put in the bobbin case. None on some Vikings, either
Looking at a lot of somewhat older manual machines, a Brother, and a Babylock, and no bottom tension. I don't remember a bottom tension on an old pink Riccar. The Brother 1500 doesn't have it, either.
Is she possibly talking about presser foot pressure? The 2100 series is electronic, but I think you could adjust the presser foot pressure on the 1400 series. The Riccar did, and so does the 1500.