I agree with 20 and 100. M means it has a main breaker. (As in your link below.)
I don't know what BW is either. Need a real old catalog.
I would read "20" as not allowing tandem breakers. "2024" would allow tandems in 4 positions. But it is an old catalog number.
Seems like UL had maximum number of poles depending on the panel rating. I think a 100A panel had a max of 20 poles and 200A had a max of 40 or
42 poles. That makes it harder to overload a panel, but doesn't work so good if you don't have much on each circuit (so you need more circuits). SquareD has panels with a lot more poles ("circuits") than that now.SquareD class CTL tandem breakers have the usual horizontal clip at the busbar end. At the gutter rail end they have what you describe as a claw, which fits into a slot in the gutter rail. The slot is only present at the positions where tandem breakers are allowed.
Your farnell link has a crappy picture for a Homeline panel on page 9 of your link. A better picture is
Regular breakers just have a clip at the gutter rail end, as with your original breakers.
The difference is only in the gutter rail.
Yes.
I don't entirely agree with the limit on circuits. If I watch the connected load I might install tandems where you are not supposed to.
I think one reason for the limit is to limit the heat produced in the panel. Heat can cause a breaker to trip at a lower current. Would seem like a 100A panel would only make 100A of heat no matter how many breakers.
There is, in general, a limitation for fuses and circuit breakers of 80% for "continuous" loads, which are loads that are on for 3 hours or more. My understanding is this is based on panel heat, and the breaker may trip at less than 100% if the load is on for more than 3 hours.
An exposed 20A breaker or fuse by itself is supposed to hold at 20A forever.