Markings only say "12D" three times, and what looks to be a date code "2-74".
It's slightly wider than 1/4 inch diameter on the top, and slightly taller than 1/4 inch tall, not counting the leads underneath. Think it's a TO-5 package. The case is connected to the base, not the collector.
It should do audio fine, but I don't know if it could do RF frequencies.