I have a follow up question from my previous question on math.SE, where I asked about the First use of little $o$ notation - for those who want to check - the answer goes back to Landau ($1909$), this could really be one of the earliest occurrences (the definition is motivated by the big $O$ notation and there is no reference to any other work, so it seems genuin).
Now I wonder when the slightly different notation of $o_p(\cdot)$ notation was first introduced. Clearly later but unfortunately there is no historically reference on Wikipedia. Does anyone have a clue or reference where I should look into?