Z31 BMC to S13, bolt on?

[DJ-of-E]

I'm actually quite confuse on the z31 BMC and the differences. I tried out the 17/16 with z32 brake booster setup and did not like the "lack of" pedal travel from the system.

I want to try 15/16 with manual booster on the S13. While I was looking up Z31 BMC, I noticed there's a 4th hole in which I do not know what it is.

On another Zilvia thread, it shows a NABCO z31 bmc with 3 openings.




However, I went to several shops and they all have "TOKICO" z31 BMC. I immediately noticed a difference.



Between the 2nd front and rear opening, there's another hole in which the stores plugged it up with something rubber. I checked the z31 FSM, but it only references the 3 flange brake lines.

Can someone explained to me what's the function of this 4th hole and is it okay to leave it alone?