Linktoform() and passing data to a referenced dropdown form

[MarLew]

Hello,
I am using the inbuilt database and I have three tables:
EQUIPMENT
CTRL_AAA
CTRL_BBB
Column SN is in EQUIPMENT and referenced in CTRL_AAA and CTRL_BBB

In my action I have this :
LINKTOFORM(SWITCH([TYPE],"AAA",CTRL_AAA,"BBB",CTRL_BBB,CTRL_AAA),SN,[SN])

When SN gets to the form it looks like this:

The SN value sent does not arrive in the Form as one of the preset references in the drop down list.
I think this must be some sort of type/format problem but I can't pin point it.
Any ideas?