Would like to know the difference between these two 4N35 optocouplers . They behave differently but cannot find what is related the "001Q" and "027Q" digit.
As commented, the different lines "001Q" and "027Q" are just date and assembly codes.
Image source: OnSemi 4N35 Optocoupler Datasheet
The two optocouplers should behave the same. If they don't, then either they are damaged, fake, being operated outside their specifications, or the difference is within specifications.
the 4n35 to my knowledge has a few "edits" or versions since the first ed, from which it has been perfected. so the newer ones are likely more "linearized" in the optical current conversion. there's only a few "edits" of the old one 4n35. the newer ones are more linearized but also suffer a narrower slope.