Your frame number is U63/344X, & I have U63/369X1 (sorry I omitted the 1 off the end of the frame number in my last post) as the frame number for the start of 1965. Then perhaps you have a 1964 frame with a 1965 engine in it?
Of course there is always a possibility that the information could have errors, however this material is usually reliable. My source is the following book - 'Villiers, singles & twins' by Roy Bacon, published by Osprey, ISBN 0 85045 486 7.
I am not able to pin down the part of the year more accurately, but it may be possible if the Excelsior factory records still exist!