I'm not familiar with what you are describing.(or I've never seen it)

Here's a picture of a CAT 3208 "TRUCK ENGINE " fuel injection pump's special fuel return line fitting/valve, that also has an "anti-siphon check valve" in its outlet. It is also used to bleed fuel filters, or bleed the front part, of this 3208 injection pump. {Note: To bleed the rear governor housing, normally, we remove or loosen the "pin timing plug" in the rear "fuel setting" cover.}