71 caps are very hard to come by! One year only style cap. I saw a NOS chrome one on here for sale a couple months ago. I think he was asking around $100. I will see if I can find it and post it here.
My 71 Demon was actually built late 70. It seems like I read somewhere they didn't use that cap even for all of 71. I got lucky and found a new Stant on eBay for like 35$ but that's been two years ago. Good luck. May want to do as said and find a 72 up filler.