Holton TR602

[YunoWho]

Hi, a friend of mine is in possession of a Holton TR602. The serial number is 848---. I have been searching all over but am unable to find when this version was made. The charts don't go to 800,000's, am I missing something? I know at some point Holton just started going for random bunches of numbers, maybe this is part of that? I don't know
Thanks