I understand you want to know which firmware is the newest.
Since the drive is being used in a commercial product, your question would have better exposure in the business forums than in the consumer forums. Reviewing the previous post, I don't see any indication that they advised to call HP.
I did pull up the HDD on the business support center using the provided model number and selected an OS to view the firmware updates available.
This was the page returned. The newest version listed is HPD9 and the previous one listed is HPD7. Given the naming convention of both listed firmware updates, I would venture that HPDA is an older naming convention and HPD6 is a newer firmware than HPDA.
I couldn't find anything that answered this specifically and this is just an educated guess, so you might want to try the question in the business forums as there are more users of the product there.
Even if you can't find the right board, it's more likely to get an answer in a wrong board over there than it is in the consumer forums.
I gave you my educated guess and that's really all I can give as I don't have access to the enterprise side of HP. The link NPW provided is a direct link to the SAS board for your product. Even if it's the wrong board, you will be more likely to get an answer in the Enterprise community than the Consumer community. You can also try this legacy board that I found here.