The matching serial numbers is not as critical as people are led to believe. It insures that Rodenstock and Schneider components to not get mixed up, and the F2 front elements do not get paired with IIC and IIc F2.8 apertures.
Cheapest bet: Buy a non-working (jammed) Retina Reflex with Schneider Xenon F2 optics. It should be cheap. Replace the front module- easy to do. Try it. Worked for me when I did the same think putting an F2 front module on a Retina IIC.
Random Ebay Example, pulled on "Retina Reflex"
If the focus is a bit off, post back. Replacing the rear module requires a spanner, rubber gromet, and patience. I've done it.
I do not have any spare F2 components, but do have a spare F2.8 front that will work. The rear modules of the F2 and F2.8 Schneider Xenons are identical. Been there, done that.