Do a part number search and you will likely find various naming conventions, but Catalytic Converter is the most frequently referenced term by Mercedes.

Here are the 2 examples you cited from mbusa


https://mbparts.mbusa.com/v-1991-mercedes-benz-350sd--base--3-4l-l6-diesel/exhaust-system--exhaust-components

https://mbparts.mbusa.com/v-1991-mercedes-benz-300d--2-5--2-5l-l5-diesel/exhaust-system--exhaust-manifold

These have a ceramic honeycomb interior which is only used for a catalyst or DPF, unlike the mufflers which are chambered or baffled.

You can always inspect yours to verify, it's state. As mentioned, the location will lead it to clogging eventually.