What you have there is pre '76 transom housing, a 1985 drive shaft housing (top half of the drive) and a gear housing (lower half of the drive) from between 1975 and 1982). Given that the you're saying 1975 in the title leads me to think the transom housing and the gear housing are original, just the drive shaft housing has been replaced at some stage. And that's not a bad thing. The MR drive shaft housings had the stronger gearset. And as it's already stamped with the ratio (1.65) that then leads me to think it was a second hand replacement, not a housing built up....
Chris....