Back in 1978 the 7807 didn't exist. Neither did Unisonic Technologies.
Please produce produce a datasheet dated 1978 or earlier showing a 7807 in a TO-3 package to support your claim.
Sorry but I am really not going to spend hours now to dig through old catalogs for you. These things aren't all available online and pretty much everyone and their grandma made copies of 78xx regulators in TO-3 cans. Even in the Eastern bloc - I still have some MA7805s made by Czechoslovak Tesla around.
Yes, practically everyone
except RCA. Looking back through their old catalogs you find they had very little to offer in the linear power IC range and as far as I am aware they never did jelly bean chips like three terminal regulators.
If it is actually a 7807 regulator it obviously wasn't made in 1978 because then that would be the part number and not date code.
I also never claimed it was an Unisonic part, only that 7V versions of the 78xx regulators do exist. The can has an obvious RCA logo, as far as I can see.
I checked a variety of RCA metal can devices and, in
all examples, the part number was arranged circularly
above the RCA logo in the 10AM to 2PM position. In the 3 to 5 position was the 4 digit date code and in the 7 to 9 position was an alpha-numeric code, which could be a mask number or manufacturing plant code. So, on an RCA marked part, there is simply no way that the 7807 code could be interpreted as a part code.
If it was a transistor then I would assume the cheap tester would identify it as one and not claim it is a thyristor (SCR).
It could easily be an SCR. RCA used to make a wide range of SCRs and TRIACS, many of them in TO-3 packages.