Rob,

Very little question your house, and probably barn, are much older than 1900. Maybe 100 years or more. The Roman numerals you mention at the joinery are "carpenter's marks", sometimes called "marriage marks". They are the marks left by a carpenter trained in "scribe rule" layout of timber frame (mortice and tenon) buildings. This ancient system is still in use in Europe today and came to America with the colonial settlers.

Around 1800 a system of layout now known as "square rule" began to replace scribe rule. This new system didn't use the archaic carpenter's marks. By the Civil War scribe rule layout was effectively extinct.

I would strongly recommend working with someone knowledgable in these searly building methods as you take on the stewardship of you homestead.

Rudy R. Christian