Chris, just to keep you interested, I have found that “EL SOL” trademark was used by an argentinian hardware store established in 1890. Up to at least 1968 this firm still existed under the managment of “Jorge A. Spilere & Cía”. It may be related but I still have no idea who made these cartridges.
I also suspect a link to Broqua & Scholberg of Rosario and Montevideo, trading as Scholberg & Broqua in Liege. However, their trade mark was the picture of a sunburst (SOL) and not, specifically, the words “EL SOL”. The SOL trademark was re-registered in about 1990 by Rubisol Ltda – run by our mutual friend in Montevideo.
Do you have any further info on “Jorge A. Spilere & Cía” ?
I also tried to link this headstamp with “Sonnenmarke” brand name registered in 1893 by Wolff & Co., Walsrode. Shotshells of this brand were imported to Argentina and Uruguay during the 1920’s but I can’t find no actual relation to “El Sol” .450 Revolver cartridges.