Accessiong the Values OnBeforeVersion AML Action="Version"

I have some problem with the OnBeforeVersion trigger, 

If I try to create a new generation through AML with e.g.

    <Item type="Part" id="XXXXX" action="version" select="id,keyed_name,description,created_on,modified_on,state,generation,config_id,item_number,major_rev,name">

and then on the server side I want to check for example if the itemnumber is still there, I can not reach that value. 

I try e.g. 

string partNumber=this.getProperty("item_number","");

and get "" back although I do not change anything through AML. 

Did I understand the action="version" wrong. 

doues anybody have any hint for me or can point me to some documentation about this trigger ?