The relay looks original. The cheapest afterglow option I found years ago was a Hueco (sp?) brand unit for a 140 SD. It has a strip fuse - OE afterglow relays have a self-resetting breaker - and a fifth small pin for a coolant temp sensor. This noticeably drops glow light time on a hot engine. I forget where else in the car a 6-pin connector is used so the upgrade looks original. Of course you need a temp sensor as well. I used a 300D 2.5 temp sensor that goes in place of the BFS 50*C switch by the thermostat (I had ditched the BFS). You can also use a 3-prong combo temp gauge and afterglow sender from a 3.5, or use one of the other three blank positions in the head with an adapter. It looks like the manifold has to come off for those options. Maybe you don't need the EDS temp sensor by the upper radiator hose fitting if you disable EGR and ARV.

Sixto
83 300SD
98 E320 wagon