Jerry,
OK. Thanks. I also saw that it was listed for use with a tape player which doesn't make any sense.
Having now taken a second look at the AIM (D55 A2) the revision record box references RPO U57 which is the tape player option. It looks like the dimensions and hole sizes for the tape player were added to the drawing. So it would appear that the screw assembly 9428499 is only used with the U57 option.
So I'm guessing that the screw I need to use in that hole would be the same as the other screws used to secure the top half to the bottom half (item 2, p/n 9421137, 8-18 x 1/2 Phillips head) which makes much more sense.
Thanks,
Bob