Most of the dial gauges I have (albeit not in Mitutoyo's league) came with both a lug back, or a flat one, and can be interchanged. All with four screws.
If you don't have a flat back in the original packaging with yours, I suspect it is fairly simple to make one. Mine are simple flat pieces (of steel or plastic depending on the guage quality) a couple of mm thick with four mounting holes round the periphery.
Take the lug back off (in a CLEAN environment), measure it up and make one.