[Tertius Noble]

I'm not sure whether or not this will be much help, but my copies of the 50 and 100 Free Organ Accompaniments are published by Alfred Publishing Co, Inc in the USA - "alfred.com" (who have been assigned all rights by the original publisher, J. Fischer & Bro.).  For what it's worth, my copies arrived within a few days of my ordering them from Amazon UK!

[16 Foot Stopped Metal]

Having spent a lot of time both with a Walker Positive and a Nicholson with an extended Pedal division, both with metal closed 16 foot - on the Walker described as a 'Sub Bass' and on the Nicholson as a 'Bourdon'; they both had metal flute ranks, so the extension to 16' was metal 'to match' the rest of the unit. For what it's worth, I have never seen wooden pipework on a Positive - and I've played lots of 'em.

More recently, I spent some years playing an extended Evans & Barr (NPOR https://www.npor.org.uk/NPORView.html?RI=D02452), which had a quite nice wooden flute unit and the 16' bottom octave was wood for a 16' Bourdon (also used for a 32' 'Sub Bass', but no extra bottom octave of pipes!).  I would say that the metal Nicholson and the wooden E&B sounded very similar, but the Walker sounded brighter, but that was really because of the brighter voicing of the Walker generally.

I suspect this may be of limited help, but my gut feel is that it is the overall composition of the stopped unit which determines the composition of the 16' bottom octave and that the tone is really all a matter of the voicing rather than the substance from which it was made.  My experience is that the tonal difference between wood and metal is much greater at 8' and above rather than at 16'.  For what it is worth, I usually find there is a much greater difference between an open 16' of wood and one of metal.