I believe that the main difference is in the casing that encloses the chip. The SECC and SECC2 are clipped onto the motherboard in a different manner.
Also SECC heatsinks are not compatible with SECC2 micro-processors.
You should have no problems using a SECC2 in a Slot 1 board.
I installed a PIII-650 on my AOpen AX6B (440BX) without a hitch.
Regards,
Terak