To answer the original question, you are missing the thin steel separator plate between the carb body and the metering plate.

http://www.ebay.com/itm/HOLLEY-MODEL-416...982&vxp=mtr

I believe it goes, from carb body outwards, Big gasket, separator plate, smaller gasket,, metering plate. I had to go to four hardware stores to find the #1 clutch head screwdriver for those screws. I thought seriously about replacing them with stainless flat socket head machine screws.

The secondary metering plate can be converted to a metering block using a kit sold by Holley or Quick Fuel, etc. Or at least one, maybe more, companies sell metering plates that use Holley screw-in jets.

The secondary metering isn't nearly as critical as primary. For one thing, it isn't as critical to nail the A/F ratio to make max power.

I like having the 4150 type (two metering block) carb even though it probably doesn't do a thing for me. But that's personal preference, not based on fact.

R.