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Douglas, ProMag calls it a TYPE-D Magazine : https://promagindustries.com/archangel/type-mag-faq/

I looks like these are proprietary. They just call them type-d. Here's the model number if you want to check them out (AA133-05). Hope this helps.

TYPE D magazines are compatible with and interchangeable among the original AA700SA / AA1500 / AAS10 Precision Short Action Stocks. Care must be taken to load the correct ammunition into the correct action or chamber Full details here https://promagindustries.com/archangel/type-mag-faq/