How might I go about exploiting Z39.50 to extract specific MARC  
records from a library catalog?

More precisely, I am trying to download sets of MARC records from  
remote library catalogs destined for a sort of union catalog. I see  
each of these records being identified with a specific string in a  
local note or local subject field. This string might be "CRRA", and my  
question is three-fold:

   1. What MARC field/subfield might I put this string?
   2. How would I go about getting the string indexed?
   3. How might I go about querying the server for records with this  
string?

For example, I might put CRRA in 599 $a. I would cross my fingers  
hoping this field gets indexes, and I might be able to search for it  
like this [1]:

  @attr 1=63 2=3 3=3 4=2 5=100 6=1 CRRA

Am I on the right track?

[1] http://www.collectionscanada.gc.ca/bath/tp-bath2.23-e.htm#b

-- 
Eric Morgan