To my knowledge, the quill size can be identical for a MTB and road stem. The difference is MTB stems will have 0 degree or upward angled rise, and road stems will typically be negative rise. Also there's a difference in the diameter of the handlebars that are compatible with each. I think MTB stems use 25.4mm (1") handlebars and road stems use 26.2mm handlebars.
your merlin is probably a 1''..not sure about the shim for the handlebar.
i had the weld on the single bolt boss on an ibis titanium stem fail once..kind of turned me off on using one bolt stems on anything i was riding seriously!