I am working on a MARC to MODS mapping for one of our journal files. Each 
journal record may contain holdings information from the 852 and from 
local holdings fields as below:

ISI Bulletin

Location: British Library
Sub-collection: DSC
Shelfmark: 4582.950000
Holdings: Vol.1, 1949- 39(2), 1987

Location: British Library
Sub-collection: STI
Shelfmark: (P)BY40-E(1)
Holdings: Vol.5, 1953-8, 1956; 1962-39(3), 1987

Is my only (or best) way to handle this in MODS to use an <extension> 
holdings schema? I have some concerns about interoperability issues if I 
use the <extension> element. For example, I know that in the UK, another 
consortium is developing its own local holdings extensions while the 
example in the MODS Guidelines is for the Z39.50 XML Holdings schema 
(which may be overly complex for these simple holdings summaries anyway). 
So I've been looking at using <location> and <physicalLocation> as 
elements for Location/Sub-collection and Shelfmark but am having problems 
linking the relevant holdings to the right location using 
<physicalDescription> <extent>.

1. Are my concerns about interoperability and the use of the <extension> 
element too great? I realise that this depends on so many things but I'd 
appreciate your thoughts...

2. If I attempt to use standard MODS, have any of you had to cater for 
similar situations or know of any examples that I could have a look at? If 
I'm going to put the data into MODS I'd like to make sure I'm putting it 
into the best possible places!

Any advice or guidance will be very gratefully received.

Many thanks,


Jan Ashton
Metadata & Standards Analyst
Bibliographic Development
The British Library
Boston Spa
W. Yorkshire