150 W HPS Ballast question

OK so I found this old ballast I bought about two years ago but can't seem to find the capacitor... but then I thought I read somewhere that not all HPS have a capacitor? or maybe it was not all MH had one... can't remember. Just wondering if anyone can shed some light on this for me.

It is a 150 W Sylvania Transformer LU150/120R says to use lamp types 150W HPS S55..

On the wiring diagram on the transformer it says "optional cap" then underneath "for HPF only"

oh and there are only 4 wires coming out of the transformer