How old is 5 shot break top hammerless Harrington and Richardson 38 S and W revolver serial number 172123?

Answer 1

This sounds like a "Defender" or a Model 925 "Defender" which is just the newer model of that same gun but somewhat different. I have NO idea based on serial of which unfortunately sorry about that.
I would guess however this early Defender made 1939 to 1946, most of these were purchase by the Government and later declared surplus and dumped on the market. The newer version was introduced in 1964 and made until 1985 or 86 when the company was sold or ? Marlin owns them now but I don't know if they bought them and Marlin is owned by Remington now and so on and so on.