[HBR] Question: Why Did the 2nd Oscillator/Mixer Shield box Disappear in the HBR-13?

[whitebear1122 at comcast.net]

I have shifted gears last week and decided to pursue the HBR-13 instead of the 12 so now I'm adjusting the dimensions on my paper parts placement diagram to accomodate the two additional 1610 Kc IF transformers and the additional 100 Kc transformer.  

  

I know that the 2nd Oscillator/Mixer was once inside a small shield box on the HBR-11/12 but suddenly was no longer needed on the HBR-13.  What changed on the HBR-13 design that allowed the crystal oscillator circuit to no longer require a shield?  

  

73, Scott WA9WFA