Sig Sauer Romeo Zero features the Shield RMS/SMS mounting standard. In this article, you can see its footprint.
Springfield XD, XD MOD.2, XD(M), XDS, Hellcat (Non-OSP) Pistol – Modular Red Dot Adapter – M.R.A - Outerimpact
Are you looking for an adapter to mount a multitude of Red Dot sights to your Springfield XD, XD MOD.2, XD(M) pistol? Look no further... The OuterImpact Red Dot Adapter/Mount answers the question!