The F61 is a small voltage regulator. The logo is that of Pressler but it came in a Standard Brand box. Research shows that Standard Brand boxes were available empty and much used for re-selling used valves.
Construction is typical of neon stabilisers with two concentric cylinders for electrodes.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 51 mm tall.
Updated April 10, 2016.