HP F series

More 21MX F series madness.... I have heard in two places now (one being Christian's post) that the E and F series mainboards were interchangeable. The HP engineering & reference documentation indicates that this was not always the case through the entire product line. There is clear mention that prior to certain date codes, there were separate mainboards for the E and F, and they were not interchangeable. After that date code, the boards were identical and interchangeable between the two. So, long story short, do not just assume the boards are the same and assume you can swap them. If they are prior date codes, you'll be disappointed ;) Now this has me curious... I wonder exactly what the difference between the E and F mainboards was that made them not compatible prior to the date code specified? I'll see if I can find further clarification. Jay