Hi Barton,
Control-number is the index on 001. 001 should have the number and 003 the MarcOrgCode, that's why it's stripped from $w for search. I don't know about OCLCs practices, so can't tell how numbers are handled there. The examples here show a number with ocm in 001:
Contains the control number assigned by the organization creating, using, or distributing the record. For interchange purposes, documentation of the structure of the control number and input conventions should be provided to exchange partners by the organization initiating the interchange.
001 #880524405## 003 CaOONL
001 ###86104385# 003 DLC
001 ocm14919759 003 OCoLC
001 #####9007496 003 DNLM
The description for $w (http://www.loc.gov/marc/
bibliographic/bd76x78x.html ) doesn't have a matching example:"System control number of the related record preceded by the MARC code, enclosed in parentheses, for the agency to which the control number applies."
Hope this helps,
Katrin