With trainers approaching 80 years old, it’s no wonder many substitutions have been made over the years. Here’s one we have seen several times. Original Blue Box trainers frequently used #6-40 screws; the most common #6 machine screw today is a #6-32. When a #6 machine screw just doesn’t fit right – it may be the wrong thread.
A similar problem occurs with small set screws; Blue Boxes often use Bristol or spline. When standard Allen wrenches don’t fit, get out a magnifying glass and see if a Bristol is in order.