The HP211 and HP211C from Tungsram are vari-μ RF pentodes that operate from battery supplies. We can not, as yet, find out what made the 1937 HP211C different from the 1934 original. Both were made with B4 or B7 base caps.
The glass is clear but little of the internal structure is clear.
The B4 Base.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter, and excluding the B4 base pins is mm tall.
Reference: 1043. Type HP211C was first introduced in 1937.