Welcome to the site.

I don't believe that such records are kept, because sandmans were produced alongside other vehicles and the sequential numbers didn't distinguish that it was the first sandman etc

However, there are several guys on here that collect data from the ID plates and could tell you how early yours is compared to others that they have on their records. That having been said, the data that has been collected is probably only the tip of the iceberg compared to all the sandmans that were originally built. So even if yours is the 1st they have recorded, it is no guarantee it was the first Hj ever, another earlier one could pop up.

Put up all the numbers off the ID plates and someone will let you know.