Collet bushing AND matching barrel - - -
That and the cruddy, billboard-style markings, were about the only difference in the Mark IV, Series 70, and the old style gun.
I got a BB variation from an Oshman's Sporting Goods in Benbrook, suburb of Fort Worth. I carried it some and shot it several hundred rounds. I was vaguely aware that it was unusualbut didn't know how much until read a column in Guns magazine which told of the new Series 70, and the rare transition variant. I believe they said there were only some 800 made, maximum, and possibly a lot fewer.
About two years later, I another at a gun show. This one was unfired, too, and I left it that way! Finally sold the unfired one in, I think, 1995, for about $1,000. (It was a 3-way transaction.) At that time, an unfired 1969 gun would have brought around $700--800 in my area.
Unfired pre-70 guns get rarer every year (DUH!) and thus more valuable. It is easy to assume that the gap between one of those and this rare variation widens accordingly. Only way to know for sure is to take it to a gun show put on by a state or regional collector's association and see what kind of serious offers you get. Check a copy of Gun List periodical and see if there's one there.
I can't believe your gun is worth less than $1,500, and probably a lot more. (Sorry--THAT IS NOT AN OFFER!
You have a nice, historical item. I can't decide if it is more or less valuable since the collet bushing was discontinued after several years production. I think MORE valuable, like the Govt Models with the Swartz safety.
Good luck to you.
Johnny