All clear as mud so far?

Prior to 1968 panels and breakers were manufactured without the rejection feature (non CTL). The rejection feature configures the panel and breakers so that the design limit of circuits can't be exceeded by adding more duplex or half wide breakers (CTL).

One example is the Cutler Hammer BR series. The panels come in 2 designs. One type is for full width single breakers only and the other can use duplex (tandem, piggyback) breakers. The panel designed for the duplex breakers has the buss slotted in the positions where the duplex breaker can be used. This can be all spaces with a circuit capacity noted as, for example, 20/40 (20 spaces, 40 circuits) or just some of the spaces, designated 30/40 (30 spaces, 40 circuits), for example.

The metal tab (in the slot that contains the buss contact) prevents the breaker from being installed on a non-slotted buss. The tab can be broken out with some effort and usually damages the housing in the process of removal. This is a type BD breaker.

A type BR duplex breaker is manufactured to allow replacements for breakers in the older panel design (non CTL type) where the circuit limiting features weren't used. These breakers will fit either the slotted or unslotted buss. The only apparent mechanism in place to try and prevent their use in the wrong place is that they are about twice the price of the breaker with the metal rejection tab.

The modification of a CTL breaker or use of non CTL breakers can result in a panel rated for 42 circuits having a total of 84 circuits installed. Yes, I've seen it at least twice.

A panel rated for 20 circuits with 20 spaces that has 10 duplex non CTL breakers installed isn't overloaded with circuits but does have equipment installed not listed for the purpose.

Other manufacturers have various methods for preventing installation of non CTL breakers in CTL panels and most are as easy to defeat as the CH BR type. Another thing to look for