the earlier tubular swing arm tacos had the serial number on the left swing arm, on the top side of the tubing, you almost cleared the paint off the location of the serial number on the left swing arm. I have seen many many many tacos that did not have a serial number as well. not all left with a serial number, some were sold un-painted, some were sold as bare frames, some were sold complete. I am sure lots of back door bikes were sold as well. Nothing in my statement is actually confirmed, just something i have come up with being one of the few who have had every year make and model taco produced under one roof at the same time to compare apples to apples. as Race said, you have a 1966 taco 44. serial number shous read 6T***