Thanks for all of the replies so far. The "M" has a square around it and is located approximately in the center of the triangle of bolt holes that I would assume would be for the engine mounts if this was fitted to a car. It is on the Distributor.

I am not sure what size truck this came out of but I was told by the seller the truck was used to pick up milk for the Warwick dairy so I would assume a full sized truck. The block does have the outlet/inlet for oil on the non starter side and there is a large canister mounted on the manifold. The oil lines are not complete.

From what I was told the truck was a right hand drive truck so I would assume it to be Australian delivered.

The Captains bars were enough for me take the risk buying this engine and it will probably be a couple of years before I will have a chance to rebuild and use it. I am mainly trying to confirm it is a 261 so I know I can stop looking for now.

I would still be interested what is the meaning of the "GR" on the engine serial number.

Thanks

Steve