This is my reamer holder. I was right- starts with a J! The memory on this one, oy….
This is the spider chuck I made. It’s a direct mount to the A1-6 spindle. That pocket on the back is tapered.
This is how I did the spider screws. Drilled/tapped the end of an Allen bolt (they are hardened, be careful) and screwed a brass screw into it then rounded it.
THIS might be of interest for the OP, at least if the Heavy 10 has a threaded spindle nose. This is how the gunsmith who had my old Logan made his spider chuck. The knurled/threaded portion is just the thread-protector for a Hardinge indexer or the very common chinese copies.
I meant to grab a pic of the spider chuck I made for the back end of the spindle but forgot… tomorrow maybe.