ysth,
With regards to the first assumption you called into question, that should have read:

Concattenated records in the output will be identified by commas in the 3rd field. This assumes no commas appear in the 3rd field prior to merging. updated

With regards to second assumption you mentioned. You are correct that since the AM only stated where the first two fields were the same that I assumed that meant they could appear anywhere in the file. That is the point of my post - to clearly state what is desired.