If my memory serves me correct..........
Most Phase Linear series two consumer products had a front panel with rounded corners. As an option, they were available with a "rack front panel" which had square corners. I am not sure if they had to be ordered with the optional rack front panel at the time of purchase or the front panel could be ordered later. The rack front panels do show up on parts lists for most of the series two products. So I am assuming that they could be ordered at a later date. Some of these rack mount front panels may have even had factory machined slots to properly mount into a EIA rack. It has been a decade since I last owned or serviced any of the consumer rack mount products. I can't recall if they ever had any countersunk/dimpled front panels/chassis. I don't recall seeing any.
The Phase Linear 700 series two amps with countersunk front panels and dimpled chassis more than likely came later in production. Perhaps around January 1981 when Phase Linear went to a fully complementary output and began introducing their pro series (black front panel) products. The rack front panel does not show up on the original 700 series two parts list. It does show up on the later 700 series two parts list when they went to a fully complementary output and used NPN and PNP output transistors (i.e. TP9054/DB15025 and NEC 2SD555/2SB600). Obviously, with the dimpled chassis a dimpled (countersunk) front panel would have come along with it and would have had to be specified at the time of purchase. I have owned and serviced a fair number of these amps.
Dean N. told me years ago who the employee was that suggest the dimpled front panel and chassis but I have long forgot. It might have been Dean himself. The purpose of this countersunk/dimpled front panel/chassis was to be able to flush mount it into a rack.
Ed
