Certain polygons of model rendered differently

Hello,

I have a 2 sided plane (sort of like a very thin box) to which I apply two different shaders (one at a time). The first one is a regular defuse shader that renders 2 sides. Next is the plane with the mentioned shader:

The second one is a “Double sided-transparent shader” which renders with problems and I have no idea of why this is happening. Here is the code of the shader:

 Shader "Custom/DoubleSidedTransparent" {
     Properties {
       _Color ("Main Color", Color) = (1,1,1,1)
       _SpecColor ("Spec Color", Color) = (1,1,1,0)
       _Emission ("Emmisive Color", Color) = (0,0,0,0)
       _Shininess ("Shininess", Range (0.1, 1)) = 0.7
       _MainTex ("Base (RGB) Trans. (Alpha)", 2D) = "white" { }
     }
  
     Category {
       ZWrite Off
       Cull Off
       Alphatest Greater 0
       Tags {Queue=Transparent}
       Blend SrcAlpha OneMinusSrcAlpha 
       SubShader {
        Material {
            Diffuse [_Color]
            Ambient [_Color]
            Shininess [_Shininess]
            Specular [_SpecColor]
            Emission [_Emission]   
        }
        Pass {
            Lighting On
            SeparateSpecular On
            SetTexture [_MainTex] {
               constantColor [_Color]
               Combine texture * primary DOUBLE, texture * constant 
            } 
        }
       } 
     }
 }

Next is an image of the plane with this transparent shader applied:

You can see in the picture certain polygons that are not rendered correctly. If it’s possible could you tell me why is this happening? I'll attach the plane model in a 3ds format in a comment since I can't add more than 2 attachments. Thanks in advance.