Don't you just love the parlance "crystal diode".  1954 was the year that Mullard took their first steps into the mass market with semiconductors.  Trumpetting their new  product as having low shunt capacities and higher rectification efficiency than those old fashioned valve things, the range started with two devices specially developed for television use.  

There were teething problems as the TV trade tended to attach these new fangled devices into TV circuits using a poker heated in t'fire thence cooking these early and not particularly robiut devices but application of tweezer heatsinks thoughfully provided by an arrangement between Mulard and Antex to solve this unforseen setback.

Type OA61 was designed as a video signal detector with it's handy PIV of 30V.

Type OA61 was designed as a DC restorative and synchronising pulse clipper with a PIV of 100V.

Both types demonstrated a minimal shunt capacitance of 0.1uF and a working temperature range of -50 - +60oC.  The future had arrived!