Numbers on baby bottles?

i'm not sure, you did not mentioned if they all of the same manufacturer. but i know that manufecturers that use several molds, some times at the same time on the same machine, mark the product with the symbole of the mold so that, in case of fault, they can follow to the faulty mold.

I believe your bottles are fine, the slightly darker colours could be from staining (I have 1 orange tinted bottle from staining), or could be the mix of plastic - any thing with a 3 or 7 should be avoided here is a quote from an article I have seen

"Identification in plastics

There are seven classes of plastics used in packaging applications. Type 7 is the catch-all "other" class, and some type 7 plastics, such as polycarbonate (sometimes identified with the letters "PC" near the recycling symbol) and epoxy resins, are made from bisphenol A monomer. When such plastics are exposed to hot liquids, bisphenol A leaches out 55 times faster than it does under normal conditions, at up to 32 ng/hour. (in an 8 ounce glass, boiling for an hour will give concentrations of 29 parts per trillion) Type 3 (PVC) can also contain bisphenol A as antioxidant in plasticizers.

Safe plastics

Types 1 (PET), 2 (HDPE), 4 (LDPE), 5 (polypropylene), and 6 (polystyrene) do not use bisphenol A during polymerization or package forming."

Read more: http://www.blogtactic.com/2009/03/bisphenol-bpa-av...

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Have you used them yet? It might just be something the company did to mark them individually. As for the slightly darker color, if they're unused I can't answer for it, but if you've been using them (especially with formula) they simply get tinted after a while. It might mean it's a good time to sterilise them and buy new nipples. As for the answerer above me: most all plastic infant bottles and goods are BPA free, so please don't freak out. Most of the time it is stated plainly on the packaging.

it's the style of the bottle