- declare a key table of "Item" elements keyed by the "Shelfmark" chlid
 - use the identity template to copy FILE 1
 - match on the "Tag" element, copy all existing attributes
 - use the "crawford-number" value as the lookup value in the
   key table of File 2 (using the third argument of the key() function)

Rather than supply the written solution, that
should be enough for you to figure things out in syntax.

I have two XML files, File 1 and File 2. They contain the XML shown below.

<Tag>852</Tag> in File 1 has a @crawford-number.
If in File 2 there is an <Item> with a
<Shelfmark> equal to the value of
@crawford-number, I want to add an @pdf-number
attribute to the <Tag>852</Tag> in File 1
containing the value of <PDF> in File 2 like so:
<Tag crawford-number="Crawford 2411." @pdf-number="016678286">852</Tag>

File 1 always contains a <Tag
@crawford>852</Tag> and File 2 always contains a
<Shelfmark>, but there may or may not be a match.

I have not a clue as to how to get started. Happy to answer any questions.
Mark

FILE 1:
<List>
    <Record>
        <Field>
            <Tag>245</Tag>
            <Data>General-Anzeiger fC<r
Philatelie.$bInternationales Insertions- Organ.</Data>
        </Field>
        <Field>
            <Tag crawford-number="Crawford 2411.">852</Tag>
            <Data>No.1-800. 10 Apr. 1883-15
Jan. 1913$aBritish Library$b5$cDPB$jCrawford
                2411.$nxxk</Data>
        </Field>
    </Record>
</List>

File 2:
<List>
     <Item>
        <PDF>016678286</PDF>
        <Shelfmark>Crawford 2411.</Shelfmark>
        <Title>General-Anzeiger fC<r Philatelie.</Title>
    </Item>
</List>

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