There used to be a separate MARC field, Field 755 - Added Entry - Physical Characteristics. It was made obsolete in 1995, and the 655 was redefined to include physical characteristics. Here is a link to the proposal to make it obsolete:
http://www.loc.gov/marc/marbi/1995/95-10.html
And a snippet from the proposal:
In meetings of the Working Group on
Form and Genre Vocabularies, which worked to reconcile the various
form/genre thesauri, members were unable to define the distinction
between field 655 and field 755 in a satisfactory manner. In the
absence of a logical distinction that holds up in all cases,
members of the group also reached the conclusion that the best
course is to eliminate field 755. The RBMS Bibliographic Standards
Committee discussed the issue in Miami in June 1994 and also came
to the conclusion that field 755 should be eliminated, as did MARBI
during its discussions in February 1995.
Liz O'Keefe