Great Britain Scott 355c 1 1/2 Penny With Black Stripes On Back

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Just posting for fun as I was please that in an old lot of British stamps I found a few of these with the Black stripes on the back. Only one 1 1/2...

Just posting the back side of the stamp...

Those are graphite lines. The graphite conducted an electric current that allowed the automatic letter facing and sorting machine to recognise whether a printed matter stamp (one line) or other stamp (two lines) was affixed. The graphite lines, soon, were replaced by phosphor bars to allow for optical identification.

Thanks NSK....great explanation of what they are and why they are there!

The graphite lines proved unsatisfactory because anything metal inside the envelope (such as a paper-clip) could confuse the machinery.

What you have here is probably SG 563, issued in 1957, or possibly SG 601, issued in 1959. (SG 601 has phosphor bands on the front, which I can't see in your picture but nevertheless might be present.)

It's worth checking for different watermarks (St Edward's Crown, Multiple Crowns, Multiple Crowns Inverted) and presence or absence of phosphor bands, because some combinations are uncommon and a few are rare. But even the cheap ones (like your 1½d) are interesting as an example of a "false start" in postal history.