It's difficult to show the selection process with such a tiny image, but I thought that some comments would at least point you in a reasonable direction.
The gray frame in the background of your image can be selected by either geometric methods (eg, using the pen tool), or on the basis of it's weak saturation and light gray tones. If I was doing this for a client, I would probably use the pen tool approach, but this would require almost completely re-drawing the grate. However, since I knew that my example would never be actually used, an alternative approach might be of interest, and performing the selection using Asiva Select would be a lot faster for me, I used that approach.
Here are the curves I used in Asiva Select. As you can see, I selected the weakly saturated areas, without blue-greens, and in lighter (but not white) tones.
This produced a mask that I used (with a tiny bit of manual clean-up) as a layer mask for a red (for visibility) solid color adjustment layer. The result isn't great, but it isn't bad considering that I started with such a tiny image.
Obviously, if there is any way you can find the original Illustrator or high resolution Photoshop file, that is the way to go, not using after-the-fact, desperation tactics like this.
Hope this is giving you some ideas.
Tom M