The Holosun HE407K-GR X2 Red Dot Reflex Sight uses a modified Shield RMSc footprint. If you have an HK VP9 optics ready model the plate listed below, which is Plate #4 might work. I would contact HK in regards to the plates compatibility with the modified footprint of the Holosun HE407K. If you do not have an optics ready model of the HK VP9 the other link below will take you to a compatible mounting plate. However, the rear lugs of the plate must be removed.
https://www.opticsplanet.com/heckler-koch-mounting-plate-1.html?_iv_code=13-RMT-MNTPLT1-50254264&_iv_fit=leupold-delta-point
https://www.opticsplanet.com/evolution-gun-works-leupold-deltapoint-pro-sight-mount-for-h-k-vp9-vp9sk-p30-p30.html
I have a VP9B optic ready and wanted to mount this sight on it. I contacted HK Parts Direct and found out they recently started making a plate to fit this sight to the VP9. Here is their email reply with link: We do have a plate available for mounting the 507K X2:
https://hkparts.net/product/hk-vp9-vp9l-optics-plate-8-holosun-407k-507k-optic-mount-steel.htm/ [hkparts.net]