I don’t remember where I found this explanation but if you’re interested in the differences between BC5x7 / BC5x8 / BC5x9 here’s what I found some time ago :
“Basically: think of transistors coming off a production line; if they have high voltage capability (low leakage at high collector voltages) they have “BC547” printed on them, if they have very low noise they have “BC549” printed on them, otherwise (so long as they work and have reasonably okay specifications) they are sold as BC548 transistors. Then there are BC546 transistors (extra high voltage) and BC550 (both high voltage and low noise). Some manufacturers then go further and add “A”, “B” or “C” to the type number depending on whether they have lowish gain (110–220) or medium gain (200–450) or high gain (420–800, or to 900 depending on whose datasheet you read).”
When I started building paper circuits I used BC5x6 (as noted on the “Paper circuits master legend”), but they didn’t work with the Rollz-5 papers and I had to desolder all of them from paper it was a total nightmare!
I later found out most Ciat circuits needs at least BC5x7, and while running out of transistor someone on the ciat lonbarde cool kids FB group explainded to me i could use 2N3904/2N3906 instead. I now use them exclusively as they are pretty common in other synth DIY designs (Lorre-mill etc.) you just have to remember to flip them around 
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There may be small audio differences between transistors but it works as planned.
