That has to be extremely rare, and I say this only from the view of a guy who has been looking for these all over for the last 5-6 years. When you find the older, more rare rounded trigger guard 639 it nearly ALWAYS has the adjustable rear sight.
Here is this tidbit though...
You can easily put any 2nd Gen fixed sight slide on any 639 no matter how it came. Thus, you would have a better chance/easier road to a round trigger guard 639 with fixed rear sight to put the top & bottom halves together than you would to go out and find an original.
I'd consider it wonderful information and a chance to learn if anyone can prove otherwise to me.
And original box/end label would be icing on the cake.