I came by this item last week on ebay.  It is a Boswell-Mikuni Racing Carburetor...38mm...based on the TM flatslide carb and modified a good bit

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Notice the engine side of the carb, with the extra piece of aluminum tube (with lots of small holes) that extends the bore of the carb by 3/4".  There are 3 sets of holes on the extension...the first row next to the carb are drilled straight in and are the size of a small pencil lead...the 3rd row is drilled straight in and is the sized of a standard pencil lead...the middle row is drilled at an angle of at least 45 degrees and is pencil lead size.
The aluminum flange (seen in the lower right) covers the entire piece and maintains a 3-4mm air space around the bore extension.   I did not get any info with it so I am not up on all of the specifics with this thing, but I think the reason for this drilled bore extension is to smooth out the air flow and thereby increasing the velocity & volume of air moved through the bore.  
When flying in the military there was a system designed for jet aircraft that kept the air flowing smoothly over the wing even when at slow air speeds and at a high angle of attack.  The air moving  closest to the wing is called the "Boundry Layer", and it is critical to keep it moving smoothly in order to create maximum lift.  This was achieved by installing a bunch of small holes along and just behind the leading edge of the wing, and then blowing air through those holes which combined with the air moving over the wing...reducing turbulance...thus helping to keep a smoooth flow of air over the wing.  
I think this perforated bore extension is doing a similar thing for the air flow in the carb....reducing turbulance and the result is a more smooth, faster air flow...and that pulls in more fuel and atomizes it better...which results in more power and better fuel economy.

WE SHALL SEE WHEN IT IS ON THE SAVAGE...THE TRUTH WILL BE REVIELED.

This carb came with an extra slide, 4 more needles, 8 main and 4 pilot jets.  The slide is modified a little (I do not know the details for that) and it uses a dual pilot jet setup in the intake side of the bore.  This puppy is ready for war!
It was said to retail for $400, and I got it for $70.99

"installing a bunch of small holes"    very funny.   What's not so funny is that it's just might be a d@mn fine carb for a hopped up savage...

You go, Bro!  I'm planning on puttin' on that 38mm Amal this weekend too.

Lancer, the aircraft was/is a F4 Phantom. I was a Crew Chief on one in the late 60's. It would fly at 65 MPH, and also supersonic. Still impressive to see at the air shows.

Tim Krantz wrote on 12/31/69 at 16:00:11:



There are a number of aircraft with the "Boundary layer air system" on them now, and probably with many that we do not know about.  It is a very effective system.

I have one of my Edelbrocks on it now, but that will be coming off in the morning.  I sold one and have been waiting the last few days to get some decent weather so I can get it off...do the carb swap...shock swap... paint prep...so much to do and so little time.  It is a terrible thing when all of the stuff you have to do keeps getting in the way of the stuff you want to do.  There just isn't enough play time.

The holey bore extension has a 38mm ID, it was machined in to the carb bore.
I forgot to mention before that the aluminum flange that fits over the holey extension has been polished to a mirror finish on the inside...that would help the air flow.

New Boswell-Mikuni mounted on Savage...will start and run but needs some dedicated jetting/tuning.  This puppy flows a lot of fuel.

This filter is 3.5" dia. and has a 20 deg angle on the flange, which helps with the fit in the Savage frame.  I just  found this one a few days ago at sudco and will be using it in the carb kits from now on.