The key for my 1970 280SL has the flimsy rubber key head. I only have the one key and for this car it fits into the locks either way whereas older keys only went in one way. I was noodling around in our favorite (ha!) auction site and found the attached chart. Seller is klassickeyz.
You can buy blanks from them, probably take it to any hardware store and have them cut it for you. I ended up buying a different product which was two metal halves that screw together with tiny Allen grubs that become the new key head but since I only have the one key, a new project will be to buy a couple of key blanks and have them cut. No affiliation with the seller but I thought the chart was very helpful.
Ok, I had saved the chart to a word document so it wont let me attach. If you are in the auction site, just go to advanced search and search on that seller. It may take a little while to find the right key blank but its there.
Hope this helps.