Hi Dave,aerodynamically speaking the body of the "missile"would mask the fins (being smaller diameter to the body)so it would be unstable in the air,it would need to be longer for the fins to have any effect to stabilise it in flight,as the fins are close to the body if thrown as a grenade I doubt they would have any effect at all other than to make it heavier,could it be a cobble that someone has put together "just because it fits",I have absolutly no idea as to what it is but it does not "look"right somehow,I hope someone can identify it as I am intrigued as to what it is,
Regards,
Don,