The 555SL refers to the frame material - Reynolds 555 SL steel.
Here are two links to this type of bike, both for sale.
http://www.shoal.net.au/~mmueller/
http://www.sandiegobiker.com/Bike%20mart.asp
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.