You can and should ask, but wait until you have seen all of the checks before you call the law firm.
There is very little chance they will even give you contact information, let alone the information you need to fill out the form. Because it's based on multiple transactions, you will be able to indicate on the CTR why you do not have the identifying information. However, if the checks are in sequence or appear to have all been written on the same day, you've got to wonder why they did not just write him a single check. Ask.
Combine the number of checks drawn on the same account with the fact that multiple offices were involved and the air fills with the aroma of structuring.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.