According George Gruhn's book, all of the National/Valco lap steels had a letter in the serial number. A few of the early ones had the letter at the END of the serial number. I had a New Yorker from 1941 that had a "G" suffix.
National made the New Yorker for almost the entire time the company was in business. They were made from the late 30's through the early 60's I believe. Fortunately, most of the Valco products are numbered in a single series, and as you stated, the one in question would be a 1955 according to the number alone. 1955 serial numbers were approx. X43000 - X57000.
The old style mic connector is confusing though... I think that those were discontinued by 1955. And the later ones had "New Yorker" printed on them. The earlier ones did not. The National "badge" on mine was multi-colored (red, black, silver), and the later ones were just two colors (black and gold, I think).
I don't think the year it was made has much to do with it's value. Condition is more important in determining value. I played a 1958 New Yorker and it had a much richer sound than my '41, so go figure! Just make sure that the pickup delivers plenty of output and that the electronics work okay and that the tuners are in good shape. It's nice to have the original case too. Best of luck.
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www.dougbsteel.com
[This message was edited by Doug Beaumier on 18 May 2001 at 03:39 PM.]