Tom: I think yours are probably from the late '60s or '70s. The older ones from the early '60s are marked like yours, except with the additional inscription "J-B7" on the right prism shoulder just above the "Nikon" inscription. I have a 232xxx like that. The later ones, like yours, dropped the "J-B7" inscription. I also have a 457xxx like that, which is pretty close in age to your 461xxx. I believe these were called the 7x35A, and they were the last ones before Nikon introduced the 7x35E, which has rubber eyecups, and no "Nippon Kogaku" inscription. According to Henry, who is probably the authority on these older porros, the 7x35E also had an updated eyepiece. Coincidentally, as Bob mentioned, these numbers seem to be model dependent. I have a 9x35 that is approximately of the same vintage as your 461xxx and my 457xxx, but the serial number is 673xxx.