Hi Ray,
The move was only for a month or so, Croydon-Evere-Croydon, so I would say it was an attachment to Evere rather than full posting.
Since I think his parent unit remained Croydon he was still technically a Home Establishment.
Usual clue to a posting versus attachment is either (A) beside the unit or a corresponding date for an assessment of Character/Conduct. The assessment was to be done just before posting out or annually.
Another reason for attachment may be given in the Trade/Qualifications section as attendance on a course or LTTB/CTTB exam after retraining.
Regards
Ross