Raven no longer stocks this "obsolete" battery (M48Z02-150PC1) but gave me the name of their provider (STMicroelectonics).  I have one on order through the distributor Arrow (it may be obsolete but they have about 3,475 in stock.)

Raven's repair people also provided me with information that the extra pin holes on the socket are left empty on the same end as the polarity mark (left end if the socket is on to the left of center on the board as you face it to install the battery.)