If an AR-10 rifle has SR-25 pattern magazine on its spec sheet, does that mean any after-market SR-25 magazine will fit into the rifle?

I am looking at Stag Arms STAG -10, which takes a tiny magazine that is not only rumored to be "California compliant" but is also rumored to be legal in Canada.  What worries me is that this rifle has some tiny mag on it that is somehow permanently fixed to the body of the rifle. I'd like to use an after-market drum magazine.   Your thoughts?

Can you take some measurements of the magwell?

I would highly doubt that Stag sells a permanently affixed mag as their default version of the gun. Of course, you could always ask them.

Or post pics or link to what you speak of.

Yes.

From Stag's website: https://www.stagarms.com/support/customer-service/compliance.html

CALIFORNIA & NEW YORK
For our customers who live in California and New York, we offer CA and New York compliant rifles. Our CA/NY rifles ship with the following:

*Cross Armory Fixed Magazine Device
*10 Round Magazine

Our compliant rifles have a the Cross Armory fixed magazine device which allows them to have features that would normally be restricted under state law such as pistol grips, collapsible stocks, and flash hiders or muzzle brakes depending on the base rifle. Our California/New York compliance rifles also ship with a 10-round magazine to satisfy CA/NY law.