ADC IC identification
You are correct about the HP part number.  The cross reference from this HP cross reference guide is shown below.  Note that it is marked as "SELECTED".  This means that it was a selected for specific characteristics, as described in the second snippet from the document shown below.

The part is apparently out of production, but there are a few of the National Semiconductor version currently on eBay.  They have a 1989 date code, but are listed as unused.

Here is a link to the datasheet for the National Semiconductor part.