Excellent Frank. I've been looking for one of these since seeing that photo of all the SEAL weapons spread out on the back cover photo of Kevin Dockery's "Special Warfare, Special Weapons" book.
If you know that image, there are two different Mecar practice grenades shown. Both have the ring type fin, like your 75mm example, but one appears to the smaller 62mm type Mecar made at the same time. So that's two types to look for.
I was lucky enough to find an HE version of the 62mm version a couple years ago, unfortunately missing the nose fuze and fairly beat up. But it is good enough until a better one comes along.
It sounds like you might have a nice rifle to hang it on. I have a 4-digit 601 Colt that is an Amnesty registered Vietnam bring-back. It REALLY needs some grenades like these to compliment it in a future display. Please let me know if you ever find another.
Rick