It baffles me how the RC-71 looses that strain-relief grommet, but out of the three RC-71s I've owned, two were lacking that part.
This is what I've used in both cases as a replacement part...hopefully this will help you in your search...it's a *little snug* of a fit, but it fits, is functionally effective, and looks similar to the factory part in the end:
The RC-71 is a recent purchase from eBay UK. It has no strain relief, but the cable is wrapped with gaffer tape inside the unit to stop it being pulled out.
We have Mouser in the UK, they stock them but, shipping charges are £12.00. A bit much just to purchase a couple of them.
We have Maplin Electronics in the UK also, where I am we have three stores. I searched for "cable strain relief" and it came back with one hit, stating "last few available". Fortunately, one of my local stores had two in stock, I purchased them, they look exactly right. There is a code on them; "KSS" & "6R1".