I think it helps channel fluid when you fill the reservoir. Don't know if you can buy a new one but it will probably be fine as is. It's probably a baffle like you say, perhaps to help control fluid sloshing back and forth during braking and acceleration.

 

Hi Odie-
Thanks, I tend to agree that it is not required. Does your 78 spider have one? The only reason I know it was there is I wanted to siphon out the old fluid from the reservoirs and replace with new brake fluid prior to hooking up my pressure bleeder so I would know there is all new fluid in the system. There must be pieces of it in the reservoir, so I'll want to get those out as well. It's pretty brittle.

 

out of interest, what is the PN of that rubber thingy?
it'll probably start with 3.3506 - *

 

it's 3.3506-0011.1