Your Anders Gustaf Leffler, b. 2/15/1756, is mentioned in the 'Bromwell Genealogy', which you can read online from the familysearch.org website by searching their library catalog; 'Title', then 'Bromwell Genealogy'. You may already have read that information but I thought I would post about it in case you hadn't.
It says that your Anders was a son of Anders (Sr.) and was a grandson of Tobias, who emigrated to Sweden about 1655. Tobias was the son of Georgius Leffler, who lived in Breslau, Schlesia, Germany.
Supposedly, this Anders (Sr.) was an inspector of the customs in Gothenburg and had eighteen children, Anders (Jr.) among them. Hope this helps.