О какой композиции DEEP PURPLE говорит здесь Роджер Гловер ?

Перевод криват. Первоисточник вот так выглядит (http://www.goldminemag.com/article/how-does-deep-purple-really-feel-about-bootlegs)

“I had a meeting with some bootleggers many years ago in Germany; we had a big discussion about bootlegs, and they said ‘Listen, bootleggers are not ripping you off; you’re not losing money because of bootleggers. The fact that other people are making money from your music is indisputable, but you’re not losing money. It’s not money out of your pocket. In fact, the people who buy these things have already bought your albums probably two or three times already.’

“It was a potent argument, and I sympathize with that. Besides, they presented me with something I’d not heard in years, which was a recording of us doing [something] for a BBC session. It was a song that was written on the spur of the moment, just a blues, very fast, and it’s great; I love it. But it was never formally written and recorded. That’s the only version of it, and I said, ‘Wow, it’s so wonderful to hear this; I’d forgotten all about it!’ So it’s through bootlegs — or, at least, bootleggers — that things like that even exist.”
- See more at: http://www.goldminemag.com/article/how-does-deep-purple-really-feel-about-bootlegs#sthash.UobqVw5i.dpuf

И Гловер что-то путает, похоже. Mandrake Root и Wring That Neck, которые можно считать быстрыми блюзами, издавались на до-гиллановских альбомах.
Возможно, он имеет в виду какой-то вообще неизвестный бутлег, имевший хождение только на ленте, но не издававшийся на виниле.
На самом полном списке виниловых бутлегов http://www.cream-revival-band.com/Deep_Purple_bootleg_vinyl_records.html я не нашел ничего подходящего.

А вообще, кому охота поковыряться - велкам ту фулл Дип Пёпл сонг лист, в нём все полуфабрикаты тоже перечислены:
http://www.thehighwaystar.com/rosas/discog/songs/

Может When a Blind man Cries?

Kneel & Pray