Handling manufacturer mergers
This is how I might approach this design... by adding an ID for the original manufacturer at the Product level, then storing the details in a history table.

The history table would allow for Manufacturer IDs (or other details) to change over time to be stored without a need for version numbers.  

Product table

Product_ID
Product_Name
Orig_Mfgr_ID

Product History table

Product_ID
ProductHistory_ID
Orig_Mfgr_ID
Mfgr_StartDate
Mfgr_EndDate
CreateDate

I would then create a view or function to pull the most recent/current manufacturer based on the purchase date of the product, rather than storing this information in the Product table itself.