I just looked and could not find.. my second choice is the ALLEN NUTS, if you can get in 5/8 or 3/4 inch.
I used to buy direct from G-W years ago , will see if I can dig up a part #, may have one in my shop somewhere, they were a tapered internal hex sprcial made chrome nut.
Another thing, they have that is nice is hardened chrome washers.. and wrenches always fit their chromed stuff.