I have an old Model 25 that I bought when I was a teenager. It has a SN of 71XXXXXX. I was told elsewhere on this forum that between 1968 and 1973 the first two digits were the year of manufacture, and 1971 sounds like a likely year I would have bought it. Then (sounds like in 73 sometime) they switched to subtracting the first two digits from 100. So it sounds like yours is definitely from 1973. They were definitely made prior to 1979.