there was thread a few years back about a silver car languishing somewhere around Dallas, sitting outdoors as I recall. For a while I figured it was 20789 which was re-bodied as a 340mm, I bought the 400 GT body that remained. It had a quick and dirty re-spray in gold but bore evidence of having been silver. I wonder if this e-bay car is the subject of the old thread.
Its a balance pipe that allows an alternate path for exhaust to flow when the exhaust pulses can otherwise stack up restricting the flow.
I thought THIS was that "outside" car, auctioned for $13K and then restored, although not 100% well. http://www.heritageclassics.com/inventory/detail/1405-ferrari-308-gtsi.html
I cannot find the old thread, I don't think a serial # was ever given anyway. The FS car you've linked also seems to have different #s on the column(35721) and tag which I can't read entirely but certainly begins with 4.
The arrogant seller appears to have become more realistic with respect to what kind of monies he stands to make on this pig (although I am certain he feels it can become a swan with $30K+ more on repairs) - he's now relisted it on eBay for $25K opening bid: 1979 Ferrari Other 400 | eBay The market will dictate what the value is - as indicated above, that's the opening bid of $25K and next to it, states - reserve not met. Happy hunting!