[rotorwrench]

I have no idea what it is but the chances of it being Mercury are about 40%. That engine would have been used on Fords and Mercury vehicles. The 410 is kind of rare. The 390 was common in cars of both Ford and Mercury. The 360 was common in Ford pickups. The production figures lean toward Ford. The chances it is a 390 are about 60/40. Those odds leave things in doubt about what it is. You may be able to measure the stroke through a spark plug hole. That's the only way you will find out what it is where it really counts.

All I was trying to get across to you is that having an M in the casting number does not mean it is from a Mercury. It certainly could have been but there is no guarantee. Ultimately, it doesn't matter. They all literally had the same set up but just in different displacements here and there with 2V and 4V options about equal.

Just to add: Back in the mid to late 60s, the Galaxie and LTD cars were very popular and many of them were FE powered. The Mercury Marquis and Monterey were also popular and very similar in build to the Galaxie series but the Galaxie series outsold the Mercury line. 1967 was the last year of the breezeway top and the first year of the Marquis. All these cars likely shared the same engine in 1967 with the exception of the 410 and 428 FE. Base cars had smallblocks but higher end cars had the FE and many were 2V.