Hi all,  I purchased a pair of AR-3's and their is a gap in the serial numbers.  I noticed that one unit has the plastic grill that slides into the grooves within the cabinet.  However, the 2nd unit has a particle board type grill material and the speaker cabinet lacks the groove found on the other.  I'm wondering if the AR factory ever utilized the AR-3a cabinet for AR-3 builds at any point?  The speakers and crossovers all seem to match, only the enclosure being different. I look forward