John Phillips mentions 'Onion Skin Paper' being used for roll
repair.  I have never heard this term before, but from his
description it sounds as if it could be the old 'Butterfly Tape'
made in England many years ago.  I still have a couple of rolls
of Butterfly Tape but it has no use today since the advent of better
archival quality paper repair tapes, such as that made by Lineco.

Butterfly Tape was a very thin paper tape with a water soluble
adhesive on one side.  The amount of adhesive was very thin so you
had to be very careful not to use too much water when applying.
It is almost transparent and has a slightly yellowed colour, not
unlike a thin layer of cells peeled off an onion (such as that used
in the school experiment to demonstrate osmosis).

Butterfly Tape used to come in a small roll, about 1 inch in width
and diameter, surrounded by a U shaped red steel clip adorned with
a butterfly.

Nicholas Simons, GB