I'm going to offer a counterpoint to the naysayers.
I got a bunch of Mays autos through the mail in the late 70's and early 80's and they look just like the one you posted. Listen, people around here will always tell you these are 'secretarial' or signed by his wife or whatever, but I don't buy it. To me, his signature has simply evolved over the years. It's important to note that when I wrote to Willie back then, it took the better part of a year if not longer to receive my SASE back. That told me that he just sorta let things pile up until he finally got around to signing them. I would think if he had someone signing things for him, they would've been pumping things out rather quickly. That's just an opinion, but I think it's valid.
Clearly his signature for the last bunch of years is remarkably different, but it appears to be a streamlined version of his old signature.
This is how he signs it now (and has signed it for many years):
196maysauto.jpg
This card isn't mine, just an ebay grab, but if you compare it to the way his signature looks now, you can see how the flamboyant strokes that were once there are now watered down to small, barely significant letters.
maysautopsa.jpg
Now look at your card. It probably is a little older than the one above, and I direct your attention to the way he wrote the 'M' in his last name. It follows the very same process, just a little more subdued.
I think he signed his name big and 'crazy' in the old days, but as time passed and the number of autographs he signed (and his bitterness grew) increased, he shortened it all to get it over with very quickly. To me, it's most notable in the 'W.' If you look at your card and then look at the 2 others I posted here, you can clearly see how the loop-de-doo he did has evolved into looking like he is writing the number 4 instead.
Also, take a look at his 1967 card. That signature doesn't look anything like what his signature is today…but it definitely has marked similarities to what you and I both have.