I see what you mean, but I don't think that could work:
That wide sliding one is a 20t, to mesh with one on the shaft below the one below it, the lower one would have to be bigger. The input shaft is on centers with the lower shaft for another 20t, so reversing it would change ratios.
So, I think a 30t pair are needed as the upper and lower shaft are on wider centers than the input to upper/lower distance.
Er, I think. I'm going to get another colchester bantam 30t changewheel the morra so I'll hopefully get it squared away.