Bearing diameters, by convention, are listed inside diameter x outside diameter x thickness. If they are listed correctly the ID is 5mm.
Bearing is a 5mm ID x 12mm OD. Unfortunately it will not fit the Alloy Bearing Block for HK 450 (SKU: 450MBB) without a bit of work so no 5 stars. The bearing OD is 12mm as advertised so it takes a 12 mm reamer to open up the 11mm bearing block housing to allow this bearing to fit. Not having a 12mm reamer but a 1/2" (12,7mm) ream I removed enough metal to allow the thrust bearing to fit. The housing is thin and needs to be reamed about 4mm deep but as thrust loads are axial it is just fine. Do not ream all the way to the bottom as the main bearing will no longer fit correctly. This is a trial and error fit. Keep the first thrust plate part of the bearing from spinning in the housing by not reaming too far if using the oversized reamer allowing the thrust half to lock up on the rounded shoulder left by reaming. Lacking ready access to a 12mm reamer, I do not know how it would fit, but I suspect much better even if the depth went a mm to far.