If I am reading this right, you are looking into an operational history of an L-19 with the serial number 22688. I don't have anything on that specific airframe, but I was able to see in that series, there was 51-12345 with a C/N of 22667. If the numbers match up in proper alignment, which they don't always do, the US military serial number would have been 51-12366. Do you have any shots of the tail of the aircraft or any of the marking prior to beginning the restoration. That might lend several clues.