Rem8nut, be advised that you show the receiver of an Astra Modelo F which is a selective fire weapon and if it is not registered in accordance with the NFA, it will be considered contraband by your friendly BATFE person.
And if it IS papered, I found: "All the guns for the Guardia Civil order were made in 1934 and 1935 (most in 1935). Total production quantity was 1126 - the basic Guardia Civil order of 1000, plus a bit of an overrun. For unexplained reasons, in June 1935 only 950 were delivered to the Guardia, and the remainder kept by Unceta. These last ones were seized by the Republicans during the Civil War."
I wonder how many spare uppers they made? Zero or fewer, I bet. The only source I can think of is a barrel/receiver extension left after a full auto lower was scrapped. And I doubt Mauser parts fit.