Eames side chairs  

First post here. I love Eames fiberglass chairs but being a cheap soul I've never found any at a price I could swallow. That changed this weekend and I'd like some help nailing down some of the details. I've read a lot on this forum but would like y'all's opinion on a few things.

At an estate sale I purchased six (6) Elephant Hide Grey side chairs. I know they are zinc H base with nice patina and that they have metal glides. From floor to lip of the seat they are 17 in tall. They have no embossed markings on the bottom, but they all do have a black circular "Zenith Prime" stamp on the bottom with a number 4 in the center. All are hand-numbered, 1-6. One has a Herman Miller patent sticker. Some (but not all) also have some number/letter codes printed in silver ink/paint on the bottom ex: "036 8205 N1" or "039 8205CN1". All screws, shock mounts, glides, etc. are original. As far as condition goes, 3 are "new in box" with a healthy, shiny, unscratched surface. Two have minor scratches, what I would think is normal wear, but still healthy looking fiberglass. Number 6 has a small one-inch crack on the right edge in the middle of the seat. All material is there, just a crack (just lol), and is otherwise perfect.

From the reading I've done on this site I have the following questions:

1. I think these are from the mid-50s, based on the glides. Is this correct?

2. Is there any concern about the lack of embossed markings?

3. What height is this? I haven't been able to find a table explaining the standard heights.

4. Is it worth repairing the cracked one? How much does that kind of thing run?

5. Is it likely that this is a numbered set from the factory or is that a "addition" by an owner?

6 I know y'all aren't a valuation service, but I have literally no idea what they're worth. Any general idea would be helpful, even sans dollar amount.. Can I let the kids sit in them? Should I buy insurance? If it helps, I know I did very well by buying all 6 for $80.

7. Any further information would be wonderful.

Thank you in advance, and I apologize for the value question.

Edit: adding pictures
<img class="wpforoimg" src=" http://d1t1u890k7d3ys.cloudfront.net/cdn/farfuture/h8lyyC5UX87SbCBMNr8GSbxFS