I've got 2 real 58s, and 2 fake 58s...from the pictures, i can say with about 95% certainty that yours is real...the extra 5% would be me holding it in my hand, the real and fake ones FEEL different...but yeh, yours looks real...

Obviously you've probably worked that out by now...but from experience, the real ones come with the velcro cable tie thing. You will also notice a slight gap where the cartridge screws in, this is because the edges are rounded, the fake ones generally get this part wrong...

Another thing that tells you straight away if its fake, it'll have a caution warranty void if removed sticker on the element when you screw off the ball. Real 58s don't have that.

The other thing would be that just under ball where it says "shure sm58", the fake ones are a sticker, the real ones are printed straight on the metal.

EDIT: yeh the impedance is 300ohm