sceniero.
Your model 666 was originally a 22/22 mag convertible, that originally had 2 cylinders.
Made from 1975-1979
Blue Book lists values as such.
100% - $145
98% - $ 125
95% - $110
90% - $ 90
80% - $ 80
70% - $ 75
60% - % 70
My Blue book is the 29th edition. The 30th edition may have higher values listed, as H&R prices have been increasing lately.
However, since your revolver is missing one of the cylinders, the value will be significantly lower than a complete model.
Note, blue book is only a guide, not set in stone values.
Jim
p,s, I forgot to mention, that I have several diffferent H&R convertible pieces myself, and I am very happy with the function of them all!!