The only clues to the date of manufacture are certain manufacturing styles that changed over the years.The color of the paint may also be a clue as HP used a variety of colors over the years.
The rounded corner case from this period is noticably smaller (see a picture) than the larger rounded corner case in my collection (see below) and has the pilot lamp above the power switch rather than to the side but the larger rounded corner case may also have been used.
Eventually I hope to prepare a chart of serial numbers to year of manufacture.
It would be of great help to me for everyone who has a vintage HP instrument to send me an email with the following information (pictures may be worth a thousand words): Model number Serial number Logo style Box style Binding post style color of original paint I will then use this information to compile a data base which I will post on this web site and refine as new data comes in.
You might also enjoy Melody Fortier's book, The Little Guide to Vintage Shopping.
I highly recommend this if you want a short, one-stop reference for vintage shopping.