I'm pretty sure this was around 1983/1984 See the papers "Modules for standard ML" 84, by Dave Macqueen, MacQueen had previously written a paper Modules for Hop

I'm pretty sure this was around 1983/1984 See the papers "Modules for standard ML" 84, by Dave Macqueen, MacQueen had previously written a paper Modules for Hope (81) which likely influenced it. Unfortunately the SML-Family doesn't have links to these in it's history section afaict. But ML languages didn't get modules until the standardization effort that produced standard ML.