After you file

@mjpb52

Glad that it was  the case that the fund families did in fact provided you, or are going to provide you, historical data on the purchases and reinvested dividends.  It was my expectation that any currently functioning fund family would likely be able to give you some data and possibly complete data.

The following instruction of the IRS may alleviate the burden of your reporting all these antique transactions for mutual fund shares.  Note that you still have to segregate and report each named fund separately.  The following is taken from IRS Pub. 550 https://www.irs.gov/pub/irs-pdf/p550.pdf and read carefully pages 45 (Cost Basis) through page 46, and see Table 4-1 on page 43.

Using Average basis.

You can use the average basis method to determine the basis of shares of stock if the shares are identical to each other, you acquired them at different prices and left them in an account with a custodian or agent, and either:

Shares are identical if they have the same CUSIP number (remainder not applicable)

If you are using the average basis method and received a Form 1099-B (or substitute statement) that shows an incorrect basis, enter "B" in column (f), enter the basis shown on Form 1099-B (or substitute statement) in column (e), and see How To Complete Form 8949, Columns (f) and (g), later. For details on making the election and figuring average basis, see section 1012, Pub. 550, and Regulations section 1.1012-1(e).