actually, to my surprise I bought Mansfield 123-5033 Stem Assembly for 8" 300 Series Wall Hydrants (OAL 11-7/8") and it worked!

I got it from amazon:

https://www.amazon.com/gp/product/B00IAY3DD4/

but I think they are out of stock for this one, but if you google Mansfield 123-5033 Stem Assembly, you'll find other places that have it. Good luck!