Can confirm no sync header or RCA jack option on back for my Castle VCO:
Syncing via the front input works great, and you input sync signals from Visual Cortex (from the Sync Generator section with H, V, Frame, Dither) or from a Cadet I (H, V, Frame).
I currently am syncing mine Castle VCO via the power distribution bus on the back just like I do with my Prismatic Rays. See here for some additional details: