Got the opportunity to see one of these for myself many months ago, just totally forgot to post the photos. Serial number has of course been obscured to protect the identity of owner of the firearm.
I think its safe to say that the Umarex (licensed KWA model) MK23 GBB bears significant resemblance to the “real steel” H&K Mark 23. Everything that has a spring just feels stiffer. Muzzle of course on the real Heckler & Koch is heavier due to material of barrel, compared to plastic outer barrel on the KWA, but the balance with magazine out of both guns is somewhat similar other than that. Scale appears 1:1 from initial glance, including many of the finer details – and the KWA is definitely not 3/4 size as in the Tokyo Marui AEP/GBB and KWA/KSC GBB MP7A1 models.