This is the straight scoop from MAC the US distributor:
"The 398 slide will fit into the 28C’s slot, but once it is in, the dome cannot lock into place. However the dome can be put back in place, and the meter used, but you would either have to use the meter “on its back”, or hold it in with a couple small pieces of tape."
So I'm still looking for a "high" (bright light) slide for the L-28C or L-28C2.