OK, I was able to discuss this with two different electrical supply houses and got a couple of opinions. First, we have to assume this was a single phase residential panel. If it is a 3 phase panel then the following ideas are moot.
One supplier, who said he had been in the breaker business 40 years and seemed knowledgeable, assured me what we were looking at was what was referred to as a “delta breaker” which was used to start 3 phase equipment from a single phase source. He said once it was running then it didn’t matter (I didn’t know that but I’ll take his word for it). He said those ITE Siemens delta breakers were over 30 years old, no longer UL approved nor were replacements available. So, if the equipment that this breaker was supplying was a 3 phase compressor or something similar then this idea may be accurate.
The other electrical supplier shrugged and said maybe, just maybe it was a Shunt Trip breaker. Remember, a shunt trip breaker can be remotely tripped via a wired signal for safety or maintenance requirements. They are normally seen in commercial applications. There were some indications that this might be a shunt trip breaker but, if so, then the configuration seemed to indicate it was a 110v shunt trip which they had not heard of.
I tend to lean toward the first explanation but I’ve still get feelers out to a couple of other knowledgable electrical contacts to see what they say.