Don't think so, I'm pretty sure that's the original FD mount. I've had three breech-mount lenses (50mm f/1.4, 100mm f/2.8 and 85-300mm f/4.5) and none of them had that switch.Im just guessing here, but it could be a FL lens? predecessor to the FD mount
It's FD.
The lever allows you to lock the lens in automatic or manual aperture mode when set at 'L'.
It's in the manual for the AE1.
http://www.mir.SPAM/rb/photography/companies/canon/fdresources/SLRs/ae1/basic6.htm
(and also on page 80 of the A-1 manual)
Perhaps it's the extremely rare Optional-L lens. When the switch is at L, a red ring appears on the front of the lens.