What is an OC 172?

[col]

I bought some OC172s and wondered what they are. They are germanuium but have 4 legs instead of 3 and I can't find the part number on the internet. The 3 legs that are together read as a pnp transistor on my multimeter but I don't know what the other leg is for. It gives an hfe reading lower than the other 3 if I test it and use that as a collector.

Col

[Paul Perry (Frostwave)]

I believe the extra leg is just joined to the case, so that you can have the transistor shielded when it is used in radio frequency applications.
Brings back memories.. from around 1962