I have seen many LM27's for sale and they all seem to have slightly different mask steps. Some have tabernacles, others don't, some look like they have a rubber gasket on the bottom suggesting that the mast might be keel stepped. Were they lots of different options offered by the factory or have people been making their own solutions?
I don't know what options, if any, were available when the boats were first built. I suspect there will have been slight variations in standard fittings during the course of the production of these boats, but that most differences will be subsequent alterations and additions.
I very much doubt that any LM27s were either built as, or subsequently changed to, being keel stepped.
That's not a definitive answer, but if you look at the internal pictures of the boats you are interested in it will be immediately obvious whether it has the standard mast support pole or a mast in the middle of the saloon. Otherwise, ask the vendor!