Hi Steve
    I have a music roll auction that is currently open, that has a
collection of about 30 Aeolian Grand Reed/Pipe organ 58 note rolls.
The paper is 10-1/8 inch wide, the holes are spaced 6 holes per inch.
They are pin end rolls and perhaps they might work on your Melville
Clark.  The first lot 166 is actually a 58 note player piano roll made
by Pianohone Co. Since it fits my Aeolian Orchestrelle Reed organ just
perfectly, I suspect if your player does take the pin end rolls, then
it probably fits also? My web site is
    http://home.earthlink.net/~rollertunes

The 58 note rolls are under the heading of Reed and pipe organ rolls.
Hope this is of help

Musically yours
Larry