The scanner did really funky things to all of my Acu-Maths, so they look a lot more stretched out than they should. Sorry.
Later Acu-Maths are pretty much just cheap plastic rules; the last ones, like the 1240 seem to be just Sterling rules with an Acu-Math stamp. The same can't be said for this one; the 511's actually an awfully nice rule. Like the 1211 and the A-51, the 511 is made of a laminate on top of some weird material. The Museum of HP Calculators describes it as a 'Dense black plastic-like material', while somebody else claimed it's Magnesium. It does seem to oxidise like Mg, but the laminate seems to protect the scales so they survive better than the (painted) Mg picketts. Anyway, it makes for a pretty nice rule. It's even got a nice cursor. Nice cheat sheets on the back, too.
See the Acu-Math 500 for the next step in this rule's evolution towards a cheesy hunk of plastic.