Can anyone point me in the right direction to get clarification on the issue of truncated account numbers on periodic statements? I found information on this regarding receipts and credit card numbers. Also found reg E 205.9(b) referring to the account number required on the statement. But no elaboration if truncated account numbers are allowed.
Any help is appreciated.

First, get yourself out of the habit of digging into the Federal Reserve's dead Regulation E. The rule belongs to the CFPB (since December 2011), and it's been amended a few times, so use the Bureau's Regulation E at 12 CFR Part 1005. The section you referred to hasn't changed since the rule was reissued by the CFPB, however.

There's one place in the same regulation where the concept of account number truncation is mentioned. Ironically, its also in section 1005.9, but it's in 1005.9(a), concerning EFT receipts at electronic terminals, including ATMs. There, we see that the receipt's identification of the account number or access device can be truncated to as few as four digits. Why the Fed (they are the folks that added that truncation option decades ago) didn't do anything about approving the truncation of the account number on statements is anyone's guess. The Fed recognized that lots of ATM receipts were being tossed out with account numbers on them, and they were concerned about it. That was the gist of the discussion about the change in the Federal Register document. They apparently didn't consider that full numbers on statements is as big a problem, perhaps because they didn't think people are as careless with statements as they are with receipts. Or they may not have thought about statements.

Anyhow, the regulation calls for the account number to be on the statement. There's nothing to say that it can be truncated. You might observe that there's nothing preventing truncation, either. You'd be right. I think the key to this question is whether you and your customer can identify which account is involved in a statement if the number is "clipped."

I'll add this: Although I cannot prove this, I am comfortable saying that no one has been cited or penalized for truncating the account number on the statement.

John,
Thank you for this information! Sometimes I dig myself a hole the more I search....