Help with Frequency Counter operation

[RB]

Hi, I recently got hold of a HP 5345A Automatic Frequency convertor plug-in
for my HP5345A Counter.
This plugin extends the range to 4Ghz.
I got no manual etc with the plugin and it shows the following behaviour

When I input a signal upto around 500Mhz straight into the plugin the
display shows correct, but when I put in a signal (from a 1.2Ghz ham radio,
the highest frequency source I currently have) it shows 394.00000 when the
input frequency is 1294.0000 Mhz

Any ideas if the plugin is fautly or do I have to interpolate the display
somehow?

Thanks