The M3 finder has several fancy prisms, including one that is a"laminated" large block that contains the permanent 50mm lines. Most of the problems you encounter with the M3 is cracked and fading lines in this prism and delaminating of the prisms, or "fungus' - which can look like a tropical growth! Most of the parts in the M3 finder are unique to this finder and as they are old now, parts are getting difficult to get, if at all possible. It is also a very costly finder to replace, around $700.