Dumb question

[El Prosecutor]

What is this part circled?
http://www.***boat.com/image_center/data/500/3252455_manifold2.jpg
Mine has a flat cover or cap on it with two bolts, one of the bolts has the head twisted off.

[fleetimus]

It is the EGR port. Later model manifold.

[blown428fe]

Maybe hot air port cover for choke on some intakes .

[Boatcop]

That looks like a Olds manifold. The opening is for the temp sender for the mechanical choke.
FWIW, the block-off plate for the fuel pump fits perfect on the opening, if you're not going to use it for the choke.
With that manifold, you'll get more ponies out of the engine if you block off the exhaust cross over. It'll keep the intake air cooler which results in better performance. Doing that requires the use of an electric shoke, rather than the mechanical one, since the exhaust temp isn't sensed by the choke actuator.

[Oldsquirt]

That looks like a "Holley Street Dominator" for a 455 Olds. Same manifold I used to run. You can tell it is an EGR-style manifold by that little black dot you see on the floor of the plenum. That is the port where the EGR gasses enter after coming through the EGR valve. The plate on that manifold should be an EGR blockoff, but I seem to recall mine having a plate and also two small core plugs blocking off some passages just nearby. Wish that were a larger pic.......
EDIT: I may have had the two core plugs in the holes that are beneath that plate. Damn memory sucks....:D

[sleekcrafter]

That looks like a "Holley Street Dominator" for a 455 Olds. Same manifold I used to run. You can tell it is an EGR-style manifold by that little black dot you see on the floor of the plenum. That is the port where the EGR gasses enter after coming through the EGR valve. The plate on that manifold should be an EGR blockoff, but I seem to recall mine having a plate and also two small core plugs blocking off some passages just nearby. Wish that were a larger pic.......
EDIT: I may have had the two core plugs in the holes that are beneath that plate. Damn memory sucks....:D
Block-offs come with some manifold gasket kits, the best solution it to get it filled in, keeps the heat out of the intake.
Sleek

[Oldsquirt]

Since it was really bugging me, I had to go find a better pic. As it turns out, Boatcop was right that that plate covers the hole normally used for a choke heater tube. The EGR ports that I was remembering from my Olds were on the opposite side of the manifold(top side in pic below). BTW, this pic is of a SB Olds manifold.
El Prosecutor, if you have a broken bolt at one end of that plate, you will eventually end up with an exhaust leak at that point as it opens directly into the exhaust crossover. With care and the right tools you should be able to remove it. If not, pull the manifold and have it removed using the EDM process. That's the best way to gaurantee no damage to the manifold.
http://i6.ebayimg.com/03/i/06/70/4f/b1_1.JPG

[Boatcop]

Damn, Old Squirt. I consider that a compliment. :D
Prosecutor, If the plate is still on and the bolt head is broken off at the top of the plate level, you can probably get the remaining stud out with a pair of vicegrips.
Soak the threaded area with a good penetrating fluid over night. Then just turn the stud back and forth about 1/4 turn, and apply more fluid. Keep that up, increasing the amount you turn it, until it turns out easily.
Be careful, since you can easily crack the aluminum.
As far as the block offs for the crossover, I used the stainless Mondello blanks. Easier than filling the heads or manifold, and just as effective.