For a short run of castings, plaster of Paris is a quick and convenient choice of materials for core boxes, it's what I used when I made my manifold. You can see them in this video.
Here is a link to what little info that exists on Hermès. http://lotus-europa.com/hermes.html I know this is not an engine performance forum but Rocco reminded me that I measured the runner volumes (CC) and there is a difference between the two. At first I thought it was poor craftsmanship but in fact it could be Hermès power secret in that different volumes could in theory broaden a torque curve. I remember some of the 1970s Trans Am builders playing around with staggered intake runner lengths. Now I have to make another silicone plug to satisfy my curiosity…and then try and cast it… I will post a picture of the one core I have.
That is somewhat unusually, usually that sort of thing is done to meet physical constraints such as space available, positioning etc. that doesn't appear be the case here so it would seem it's a deliberate design choice. As I said earlier, manifold design is always a balance of comprises, unequal runner volume would probably reduce the peak performance somewhat but at the same time it may, as you've suggested, also broaden the power curve, it's a comprise they chose to make, sacrifice a little top end power in favour of improved driveability, a very reasonable choice for street driven car.
