Although holding by the Ba'alay Tosafot, the RaN (1320-1380) admitted the following:
 

The RaN, (Shabboth 33b “Hilchath”) citing the Rambam, writes that when the skin is being processed it is split into two. The section that is stuck to the flesh is called "qlaf" and the writing is done on the side that is facing the flesh itself. The section that is toward the hair is called "duchsustus" and the writing is done on the side that is facing the hair.

He (the RaN) further writes that this is the opinion of the Ramban and the Rashba in a response (See below).

The Ran also writes that according to this opinion, those that write Tefilin must meticulously scrape off the skins toward the side of the hair so that what will remain will be considered within the category of Qlaf.

This definition of Qlaf is found in responsa numbers 137,  139,  153,  and 239.  Also, the current practice of the Yemenite community as described to this author is in accordance of the RaN's version of the Rambam.  The version of the Yad HaChazakah found in the Boedlean Library in Oxford University, England, which was written by the Rambam's own hand has the RaN's version.