In my opinion this appears to be your part number 58-178. The height is 5/8" and it has two barbs on one side at the top. See my pictures.
The label on the Pella window sill reads: PELLA - 012810 C0710926303 NFRC PEL-N-014
If you believe I need your 58-178 seal, I will order it.
Thank you,
Mark
Paul from SWISCO responded:
October 28, 2014
Hey, Mark. I agree, the 68-178 looks like a close match. Just make sure the dimensions all line up.