[Sgt Hulka]

The sub panel is a Siemens rated for 100 amps fed from the main panel via a 50 amp double breaker. I'll have to make sure I don't overload it by transferring those 2 circuits into it since it does have 2 openings available.

 

[seharper]

According to the panel wiring diagram & label on the panel door type A or duplex are not allowed it says type C breaker only.

No it doesn't. The part of the diagram you show in message 20 is not listing the total list of usable breakers. It is only showing type C as an example, it's showing "When you see this in the drawing that means a Type C not a Type A".

The canonical list of accepted breakers is in the 5th box in your image at message 11. "Use Challenger circuit breaker Type A, C, HC" + 3 more lines of text. That's the line that matters.

I've seen panels where tandems are not allowed, and the tandem models are explicitly excluded from that list.

All the bus bar stabs are solid with no notches at all.

Search the labeling for "CTL". I gave it a quick browse and didn't see it mentioned anywhere. That would simply mean it's a pre-CTL panel, and you wouldn't expect to see notches. The notches were only added post-CTL as a lockout device to exclude tandems from where they didn't belong. However CTL has been repealed.

Regardless they kept selling non-CTL breakers like Type BRD/A which did not care about the notches, to support pre-CTL panels.

CTL was Circuit Total Limitation, which forbid panels from having more than 42 circuits in the panel. This was repealed I want to say in NEC 2011. NFPA is pretty honest, and when the body of accident data reveals that a rule was unnecessary and stupid, they repeal the rule. Happens all the time. So the only barrier to tandems everywhere is manufacturers wanting to hold back "the right to use tandems" as a premium feature you pay extra for. However, that is not Eaton's marketing style, so they had UL approve all Eaton panels for tandems. How far back that goes into Eaton's legacy, I don't know.

 

[Sgt Hulka]

This is very confusing to say the least. I did find a white UL sticker at the bottom of the can just below the bus bar. It says CTL. I also read that 100 amp service panels were limited to 20 breakers. Did this change?