Focus Transmission Failure - Compatibility Question on Part Numbers

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If someone out there knows some finer details about Focus Auto Trannys please Help! Help! Help! Wife's 2006 (Apr) 2.0L auto focus with 89.000km on the clock is now in a transmission shop due to going into "fail safe" mode and "transmission malfunction" indicator light coming on. Box pulled down to find metal on the pan magnet which has come from a bearing failure in the converter (or so I'm told). Advised shift solenoids will all need to be replaced as they are contaminated with metal from the bearing failure.....this was the cause of the initial warning light, apparently......Pretty peeved as the auto had actually been serviced regularly (new oil and filter 20K previous) despite what the service manual says.

Long story-short, rebuild/exchange converter and full set of solenoids plus other required parts = drive-away price of $5350 !!!! Nice one for a car probably only worth a little over $10k on the current market.

After picking myself up off the floor have promptly started looking for a suitable second-handy. Have found a low KM unit for a reasonable price interstate ($650 + $150 freight with 3 months parts & labour warranty). Box is supposedly out of a late 2005 model so assured that it should be fine as its the same model, HOWEVER.........

Checked transmission model numbers (stickers on the boxes) on mine and compared. Found one letter difference:

Mine: 5M5P-7000-AB PVAG
Replacement: 5M5P-7000-AA PVAG

Can anyone help in telling me what this slight difference is about? (AA instead of AB). Net search tells me that both boxes where put into 2.0Litre cars in Britain, but why? Was the AB box just a slightly updated version of the AA..............

Don't want to ship interstate, spend money having it fitted to find it doesn't work or there is some compatibility problem or other.

Anyone got any ideas????????????