Frisian frs vs. stq

Before 2006 there was only one language tag for the Frisian languages, fy. 2005-11-16 ISO 639.2 identified fy as Western Frisian, adding frr for Northern Frisian, and frs for Eastern Frisian. So far it is clear, ignoring the detail that Northern Frisian consists of several rather different dialects.
 
After the publication of RFC 4646 the IANA language subtag registry now contains fy, frr, and frs. But not fry, this is an alpha-3 alias of fy, and the RFC 4646 rules are roughly that shorter alpha-2 ISO 639 codes win.
 
It is not obvious what the Y, R, and S in these subtags stand for, one plausible theory is that S stands for Seeltersk, the Frisian language spoken in Saterland; see the S marker on the map.
 
Interestingly ISO 639-3 uses stq instead of frs for this language, this is also the code used for the stq.wikipedia. Two alpha-3 ISO 639 codes for one language are rather strange, and there is another theory to explain this situation: frs is the code of the Lower Saxon nds dialect spoken in Eastern Frisia, the territory including Aurich at the A marker on the map. In that case the S might stand for Saxon.

The ISO 639-2 rules make it fairly difficult to get codes for nonsense, there must be some evidence what they had in mind when frs was registered. OTOH with codes such as eur and tlh Occam's razor might miss the point.