This type represents a log sequence number in Postgres as defined here

/*
 * For historical reasons, the 64-bit LSN value is stored as two 32-bit
 * values.
 */
typedef struct
{
    uint32      xlogid;         /* high bits */
    uint32      xrecoff;        /* low bits */
} PageXLogRecPtr;

the comment above it says:

For historical reasons, the 64-bit LSN value is stored as two 32-bit values.

I am curious what the historical reasons are considering that the getter/setter methods basically treat this type as an uint64:

   /*
    * Pointer to a location in the XLOG.  These pointers are 64 bits wide,
    * because we don't want them ever to overflow.
    */
   typedef uint64 XLogRecPtr;

   static inline XLogRecPtr PageXLogRecPtrGet(PageXLogRecPtr val)
   {
       return (uint64) val.xlogid << 32 | val.xrecoff;
   }

   #define PageXLogRecPtrSet(ptr, lsn) 
       ((ptr).xlogid = (uint32) ((lsn) >> 32), (ptr).xrecoff = (uint32) (lsn))

The only reason I can image is that there was no uint64 type back in the day…