I've learned (via forum discussions, Ebay monitoring, etc.) that these Cherryburst (CB) Rippers are indeed very rare. However, no one seemed to know exactly how many were manufactured, since Gibson had failed to document CB as an option for the Ripper. 

The most well known and oft-cited example of a CB Ripper is the bass that was played by Mark Evans, former bassist with AC/DC, from 1975 through 1976-7. I was able to contact Mark through his website and he shared what he knew, which turned out to be significant:

You may know this already but Gibson made only 6 Cherrybursts ever, two of which came out here to
Australia, as to the other 4, I have no idea... They kept the others in the US and used one in the Ripper display ( as an finish option ) at the Namm show in 1975 where the Australian Gibson people saw them and ordered two. It was a standard jibe that they were the only people at the show that had any interest in the Cherryburst. 

These distribution numbers make sense.  I can't speak for the whereabouts of the two Australian basses, but I believe I can account for the 4 basses that remained in the States:

• I saw a big burst for sale about 6 or 7 years ago on Ebay.  It located in New Jersey at a Guitar Center and was listed with an $800 Buy It Now price.  It was in rough but playable condition and had a red tort pickguard that was broken at the output jack.  It was eventually sold.

• Steve Wood has a real beauty, also with a red tort guard. The smaller burst pattern on Steve's bass is the same as on Mark Evans' Ripper, though Mark's bass did not have the bright red tort pickguard.

• Believe it or not, I've owned two of these.  The first one was a crying shame.  Serial #186372. It was refinned, butchered, and beat to hell, but the control cavity contained the original paint...CB without question, though the burst pattern is difficult to determine. I wound up selling it to a friend who has since passed it on to someone else. 

• The big burst bass I have now came from Texas and it too was a basket case.  Serial #102562 Broken truss rod at the nut, frets pulled out, neck pup and pickguard swapped out.  I bought another basket case '74 and now have a complete bass.

It's worth noting that these four are the only CB Rippers I've been able to find in almost ten years of extensive searching.  This makes for a total of five distinct sightings, counting Mark's bass. All of these basses are of maple construction and have the larger 1973-4 body contour.  At this point, I'm inclined to believe that these were a trial run intended to test the marketability of CB as an option.  It tanked at NAMM and so was shelved.