The stock clear filter holder at the back of the lens actually holds gels. I bought a sheet of .9 and .6 for about $5 and cut the right shape out.
That's what was used here.
I also have the screw-in filter holder which uses 58mm filters. There is not a front element filter available for these big lenses. Would be insanely expensive. (As if the lenses were reasonably priced!)
Ah okay, I understand now, thanks.