Can I measure 50/125 µm fiber with the DSP-FTA410/420S?

Yes.
 
The DSP-FTA410S comes with a set of 62.5/125 m multimode leads, which can give it the impression of being able to test just 62.5/125 µm multimode. This is not the case.  We ship The DSP-FTA410S with 62.5/125 µm multimode leads since that is the most popular size although there is a move to 50 /125 µm multimode within the industry. If you wish to test 50/125 µm multimode, all you need is a quality set of 50/125 µm multimode leads.