For what it's worth, here is Ryuji Suzuki's take on what is in Microfine, copied from his now-defunct website:

Microfine is a ultrafine grained developer supplied in dry powder package. This
formula somewhat resembles a cross of D-25 and Microdol. The developing
agent is metol (3-7%). The solution contains a large amount of sodium sulfite
(70-90%), some sodium chloride (7-15%), and a small amount of sodium
phosphate monobasic (0.5-1.5%) to make the solution pH very weakly alkaline.