"if there was only one transaction into or our of the IRA account (that one transaction being that one-time rollover), there would never have been an 8606, and the only record of the after-tax basis would have been the 1099-R generated in the year that the rollover occurred."
Following the instructions on Form 8606, yes, that would generally be the case. However, sometimes someone will file Form 8606 to only record the adjustment to basis even though the instructions for the form do not say to do this.
However, basis in traditional IRAs belongs to the individual, not to any particular traditional IRA account. If your brother had other traditional IRAs from which distributions were taken in or after the year of the rollover, some basis was distributed. All of an individual's traditional IRAs are treated as one combined IRA for the purpose of applying basis to distributions.
Likely the best (and possibly only) place to find out how much after-tax basis was distributed from the 401(k) would probably be the 3rd party investment company that administered the employer's 401(k) plan. The IRS would have also received a copy of this Form 1099-R, but Wage & Income transcripts that would include the details from this form are likely only available from the IRS for 10 years. The IRS suggests contacting the payer first. See Form 4506-T:
https://www.irs.gov/pub/irs-pdf/f4506t.pdf
