With out handling it, which is the only sure way to tell, my best guess is it is a costume stomacher made in the late 19th C/earlier 20th C for stage when handstitching for stage was still very much in vogue.
But I agree the trim , color range or quality of the stitching are not 18th C., more 1880s - 90s.
I have handled some stage costumes from the 1890s - 1910s that were as well made as any fashion dress, so it's quite possible.
Hollis