Welcome to the insanity that is owning a sandcast! Ha!
All kidding aside, congrats on getting one of these special bikes.
Your bike shipped with a wrinkle tank in 1969, so indeed it should have one to be "correct". Wrinkle tanks go for anywhere between $400 (in bad condition) to $2000 or more under the right circumstances. I suspect that a restored bike that doesn't have a wrinkle tank is actually dinged a bit more than the value of the wrinkle tank simply because of the perception that it is not correct. Just a guess.
A few suggestions:
1. Camp out on eBay and wait for a wrinkle tank to come up. Have it painted and you will be golden.
2. Buy a reproduction wrinkle tank from Yamiya. Of lower value...but much cheaper than an original. At least that way you can point to the wrinkles when someone asks
![Smiley :)](https://cb750sandcastonly.com/smf_forum/Smileys/default/smiley.gif)
3. Don't worry about it unless you really need to sell the bike for maximum value. You can't tell your bike has a later tank until you go to fill the tank with fuel...which means you are riding the bike...which is the point after all!