That's not a Spratling mark, it's an A and E inside a heartshaped M with a DF above it for Distrito Federal which is Mexico City. There may be an eagle 23 with it as well. It's an unidentified maker.
Hi,
I don't think it is likely that this is a Popowski design. Although they remain unknown, AEM was a productive firm that had a varied output. They did produce Popowski designs, but much else besides. All of the pieces made by them that are marked Popowski have a distinctive modernist, usually abstract graphic quality. Stella Popowski was a painter first, a jewelry designer second, her pieces were usually mixed metal sandwiched with black resin and imbedded with stone inlay, this technique gave her a "color palette" for her painterly designs. If you do a web search you will find examples of her work that demonstrate this.
I see that mistake made frequently, the Hougart book info for the AEM mark says: AE, inside a heart shaped M. Eagle 23 (also seen on Rancho Alegre and on popowski) so people take it as Popowski even though the pieces by her that I've seen also had her popowski mark, which this doesn't.
It's the same kind of thing I see with the conjoined JF mark, because this site says the work is derivative of Margot people list it as being Margot not knowing what derivative means.