Hello,
The front suspension on these cars has rubber mountings (subframe mounts). The front suspension actually "floats" on these rubber mounts. The support rod you need keeps the lateral movement of the subframe in check . It is the "rigid" connection between the "floating" subframe and the chassis. It is not a safety issue, since the front suspension is also bolted through the rubber sub frame mounts. However it will tighten up the handling and feel of the front suspension a bit.
I heard these were becoming NLA ? The same item was used on all W110, W111, W108 and W109 Mercedes sedans coupes and cabs of the era 1963 through 1972. Here again this would be a easy item for a supplier to reproduce if it did become extinct.
The more common failure in northern climates, is that they rust and break into two parts.