For slugs, I would simply pin gauge that threaded collar and determine whether it is larger than .729”, ideally a bit larger still, such it would freely let the slug pass through untouched. Looking at it and speculating the reason for its creation, I would BET it is larger diameter than any of the choke tubes, and made to be a thread protector for firing slugs - such the shotgun is effectively a cylinder bore choke right now.
Undoubtedly, that collar should thread out - it may be stuck, but it’s nothing more than a lightbulb, threaded into place. You’ll likely need to add some heat, and certainly some penetrating oil, since the thin shroud steel tends to hug pretty tightly onto the choke collars.
If it’s large enough diameter, or if you are willing to ream it to be large enough, I would consider that collar to be your “slug choke,” then the short is ~Improved, Medium Range is ~Modified, and Long Range is ~Full.
Originally, my JCH 20 only had an Xtra Long Range choke, an X Full, but I procured a few others over the years. I wouldn’t do so today, but I slug and buckshot hunted with mine for many years without a choke tube installed - I packed the threads with hardening grease to limit their fouling. Since they’re about an inch in ID, I never did get any contact on the threads, and the grease did well to stop powder fouling. Given a lathe at the time, I’d have turned a simple collar like that pictured here as an internal thread protector.