Bill, it looks like the old Murphy muffler. No kidding , really. Available late 70's early 80's or somewhere in that time frame. Never used one so I can't report how they performed.
Dennis

Page 9 of this link -

http://docs.google.com/viewer?a=v&q=cache:hmKituCeWYsJ:www.pdarcs.com.au/index2.php%3Foption%3Dcom_docman%26task%3Ddoc_view%26gid%3D58%26Itemid%3D95+murphy+muffler+control+line&hl=en&pid=bl&srcid=ADGEEShVqPODTNZ0LcavJAyizq3wMJKcIFi4jodJ8WMzCn_02-VMEpwKSPXHO3LeEzYBf14OCIXEPv9j0oxxy-nL4Hl62wK2Nt81LTk60KzQxylPeXhJ2Wlg_llkYABUVYtGA5wgoSkl&sig=AHIEtbTuWj3X1m2c9cF6RE_cDv0kyLOhHg

And here with post 60 -

http://www.rcuniverse.com/forum/m_6586778/anchors_6897822/mpage_3/key_/anchor/tm.htm#6897822

Sorry, no pix, no ad copy, but...

As I recall the Murphy Muffler from the 1960's/70's? - it had slits on the mid-block - there may be such slits in the bottoms of the 'fins' in the pic, but I doubt it. The Murphy's mid-piece 'block' was not that large. There were no outlet holes at the end furthest from the exhaust stack...

The mid-section of the Murphy Muffler was covered by a not-quite-slack, elastic belt and all the exhaust flow was beneath the belt/above the slits in the aluminum core. You know that party favor that produces the sound of a LOUD f(latulence), the one that looks like a sausage balloon with the end cut off? In a way, that was the sound suppression principle involved: the pressure to 'lift' the belt off the core muted the sound pressure level. Sort of like a reed valve pre-loaded to restrict flow?

I think I heard one in use, way back then, and it did - sort of - sound like that party favor. If you don't mind your (otherwise stupid) engine giving you the Bronx Cheer, it supposedly brought the noise level down some...

But, back to the GMA-mod engine - I expect that the 'muffler/exhaust extension' merely has cooling fins, and exhaust gases only leave the gizmo though the holes in the outer face. Not much different from today's "chip" or "tongue"  ""mufflers"", except the holes were on the outer face, instead of being a bank of small diameter holes on one of the flat faces...

Back about then, DuBro also sold a similar "muffler" - a stack of iron plates about the shape of the exhaust stack. They had square 'ports' in them that did not line up exactly (ports like the 'windows' for exhaust ports in a ringed engine). Alternating 'plates' were 'full open.' The final 'plate' was blank, with no holes other than the mounting bolt holes.

Back then, almost all fuel, and all respectable fuel, was castor oil only. The DuBro 'muffler' quickly 'carboned- up' and restricted the engine severely. Added: another regular maintenance item: undo the muffler plates and de-carbon frequently. Sound reduction was nothing to brag about, either... ...and no way to tap for muffler pressure...