Yep, definitely a Marcus Magazine Adapters conversion. The mags may have been done by Marcus, too. If you have any questions or just want to see where it came from, look in the Vendors section of the 'Files' main forum page, "Marcus Magazine Adapters". Great guy and does quality work!
And, yes. They are all aftermarket SKS duckbill mags that have been converted to use on that system. You would have to remove the adapter to fit regular SKS mags again.
Try the mags you have before you take off the adapter. They work great. And you already have 3 mags.