Propane or LPG fuel systems are manufactured world-wide.  Each manufacturer has a target market and, for most, it is the fleet market as this is where the largest volume of sales would typically be.  Impco was one of the early system manufacturers and they still make products for older carbureted vehicles.  There were a number of other manufacturers that appeared at the time like Century and Beam but these have either gone out of business or have been absorbed by larger companies such as Impco.  Some, like Holley, were only in the business for a short time.