Per my posts #610, #613, and #623 on the BA American Express Companion Voucher thread, and thanks to Globaliser whose questions prompted me to look at the issue in more detail: it appears there is a bug when selecting an 'old style' BA American Express companion voucher and searching for First availability.

Problem:

• The option to select "First" as a class on the search page doesn't exist; it still exists if no voucher is selected or if a 'new style' voucher is selected (images in post #610 on the other thread).
• This gives the misleading impression that you cannot use the old-style vouchers for a First redemption.

Workaround (from end of post #623 in the other thread)

• Select your old-style Companion voucher
• Leave the Class as one of "Economy", "Premium Economy", or "Business" (it doesn't matter which). On the results page, you will only see economy, premium economy or business options, so the next step is to...
• ... select Change (next to the Outbound date near the top of the page). This gives a modal dialogue that lets you change the origin, destination, dates... and includes a drop-down box for class, that includes First as one of the options.
• Select First from that drop-down box, leaving everything else as before, then click Find Flights.
• The results page will now display results for Premium Economy, Business, and First.