Please don't think these two species are interchangeable! Lavandula stoechas can be toxic. Its very high in ketones and camphor and has a very different chemistry than L. angustifolia (syn. L. officinalis, syn. L. vera) (which is the variety that should be used therapeutically).
You can view the scanned pages from Geunther on Stoechas with the various constituents at my blog at https://sherwoodlavender.wordpress.com/. If you'd like to look at L. angustifolia let me know.