This is the Black Dog 26 round .22LR magazine for Olympic Arms format .22 AR 15 uppers, including the Bremmer Arms upper and the US military M261 conversion.
Manufactured from highly durable polycarbonate it is fully strippable for cleaning.
Please note there is some variance between M261 conversions which can mean the mag won't quite lock in on some setups due to clearance of the feed lips. If you experience this you can easily modify the magazine by filing a small notch just in front of the feed lips to allow clearance.
See the example photo of the modification from Black Dog, an easy job with a swiss file or flat file, do not use a dremel.