The only place to install a strap button on a Harmony Sovereign is the upper bout.
Never install a strap button anywhere on the neck heel. There just is not sufficient wood to carry the load of the screw, which tends to split, and the guitar.
You can see below there is a good inch of unsupported cross grain wood in the heel. Harmony Sovereign guitars have the thinnest wood across the dovetail of any guitar I have encountered. Wish I had a dollar for every broken neck heel caused by improper strap button installation.
Please note: We install strap buttons on request for free on any Sovereign guitar. Free!!