What I stated came directly from the SCSW 4th edition. Any M29 made after 1979 won't have a 6-1/2" barrel, it will be 6" unless you get a 4" or 8-3/8", or the ones that ARE rare, a 5" or 5-1/2", or a non-catalog special order length. To quote the catalog, top of page 232, right hand column, 6-1/2" barrel may bring 10% premium, even though 6" is more scarce. All that means is there are more 6-1/2" barrels than 6" barrels, not that the 6" barrels are rare. To further quote the catalog, also page 232, further down the right hand column under the engineering changes, M29-2 -1979 6-1/2" barrel length dropped in favor of 6" as standard barrel length. "Standard" would imply that if you didn't order the gun with an 8-3/8" or 4", or a non-catalog length, you got a 6" barrel.
I have a M29-2 with a 6" barrel. It is part of a commemorative series of 1080 guns, most of which have 6" barrels (the remaining ones have 6-1/2" barrels). The reason for the mix of barrels is that the commemorative was released in 1979, the year the 6-1/2" barrel was discontinued, and this commemorative series used the remaining inventory of 6-1/2" barrels. In this series, there are more of the 6" barrels by far than 6-1/2" according to collectors, but I've never been able to find out the precise number of each. As for the number of 6" barrels in all M29 production, I know of nothing outside of company records that would have that info, and those records probably no longer exist.