With trepidation, I jump in.
I don’t see where you see the explicit instruction to use 650. As a subject term this is coded 150 in the MARC Format for Authority Data (MFAD). We also code place names as 151 but use them as X10 in the MARC Format for Bibliographic Data (MFBD) when they are jurisdictions. I don’t think we can use MFAD coding as an instruction, if that is how you are interpreting explicit.
If you are seeing something differently, I would be very interested to know. I think many of us have very different interpretations owing in large part to the lack of any organized instruction on the use of LCSH in general.
The 680 says "$i Here are entered books set in a type size larger than normal for the benefit of persons with impaired vision, as well as works about such books.” To me, nothing here says to code it in any particular way. In fact, I would argue this says “hey use me as a genre or a topic!”.
I disagree very much that 650 #0 Large type books. and 655 #0 Large type books. are redundant. They are not. 650 in the MFBD means topical term. 655 in MFBD means genre term. They define to very different types of structured data. The fact that the terms are identical is incidental. That fact that most of our ILSs are too “dumb” to differentiate between topic and genre is not a reason to not code as 655.
It drives me nuts to see, for example, 650 #0 Science fiction, American. on an actual science fiction book when that book is not about science fiction in any way. If it is a work of fiction, any 650 #0 in that record should have $v Fiction. applied.
From: John Lavalie <[log in to unmask]>
Reply-To: Program Cataloging <[log in to unmask]>
Date: Wednesday, February 25, 2015 at 11:21 AM
To: Program Cataloging <[log in to unmask]>
Subject: Re: [PCCLIST] Large type books as a genre
So, if appropriate, a term such as “Large type books”, which exists in LCSH, can be coded in 655 in a bibliographic record, with second indicator “0” indicating that the term comes from LCSH:
655 _0 Large type books.
This is true of any form/genre term found in LCSH.
But the authority record for large type books explicity says to use 650 in this case, so 655 0 would be redundant.