I think I have it now.
The level layouts and enemy configuration stay the same at all difficulty levels, the only thing which changes is the sart time.
World 1 Level 1:
Easy = 30s
Medium = 22s
Hard = 25s
Hardest = 20
I think we can take it that MAME (my version, at least) has the dips wrong and that medium and hard are reversed, in which case I was using the correct difficulty setting originally and my score of 127,280 stands.
The score of 148,100 is not legitimate for the purposes of this challenge.