I think you mean you have a Pintail Lightweight model. However, the 1968 version of the David Nuuhiwa Lightweight model often came with a pintail too, more and more so as 1968 progressed. Either way, #9575 was not one of later numbers in either series; it was actually more like one of the earlier numbers, given that your board was made in March 1968. Both models were made up until the end of summer/early fall of 1968.
We'll have Bing look up your board in his old order book and see who it was made for.
If you want to send us some photos of your board, I can probably tell you a lot more about it and how it compares to all the others I've seen. You can send your photos to me at email@example.com .