Lucky me, I was asking at a small city post office and the lady said "HERE" and handed me a template. I had been asking, can I order one, at different locations for some time. I've lost track, but supposedly this is something you can request from USPS.

https://pe.usps.com/text/qsg300/Q201.htm

Most of my confusion was caused by taking the envelope inside, which now I know better!

I only left the card part on there because I sometimes make and mail postcards. Interesting that there are some strict size regulations for Postcards. 6x9 is pretty large, I can understand the 3 1/2 x 5 might start to be too small for handling. The National Postcard Week last year, at least one person, put the address the short way and had postage due or other issues for their mailing. I never would have known that regulation was important.

I don't know if this one prints the right size? I didn't test it. Looks like it would need to be on legal size paper and the 1/4" slot wouldn't be much use. https://www.printcopyfactory.com/uploads/images/USPS%20Notice3A.pdf

The USPS form (the template with a 1/4 slot) we are looking for is 1050251 mine is Rev H and says 2008 on it. It is 8 x 13 This is what the clerk will bring out, to check the envelope, if you go to the counter.