Here’s what the (cracked) faceplate of the FC1002 Frequency Meter looks like, through polarizing filters that reveal the internal stress.
A circular polarizer screwed on the lens:
A sheet of linear polarizing film held in front of the lens:
For reference, none of the other instrument faceplates on the bench show anything other than uniform gray, with one exception that points directly to the plastic injection point.
I’d say this plate cracked due to unrelieved internal stresses and not anything I did or didn’t do.
The Smell of Molten Projects in the Morning 
I believe that in many cases properly manufactured plastic pieces such as this one should have been tempered. Of course it is material dependent and most of the time the devices don’t live long enough for these things to be an issue. However, depending on the environment they live in and if the item is taken care of, many of us do get to find out.
When I think of tempering that sheet, I see it on a hot plate curling up like a potato chip!
Most likely, you’d confine it between two warm plates until it relaxes…
You just need to break down and buy yourself a NEW frequency counter:
http://www.ebay.com/itm/Unbuilt-Heathkit-Im2400-Handheld-512-MHz-Frequency-Counter-/171225511028
But, but, but … what fun is that? [grin]
I wish the risen-from-the-ashes Heathkit Company all the best, but IMO the days of hardware kit building are just plain over.
I built a lot of Heathkits over the years (should have kept the H8 computer [grin]), but I started to see the writing on the wall in the early 80s. As surface mount devices became more important and IC pin counts went up, the approaches Heath used weren’t going to be viable. Prebuilding a lot of the kit (IIRC, the CPU board in the computers was completely done) was the big flag to me. By the end, it cost more to do a kit than to buy a cheaper and likely more reliable item from Boxes-‘r-Us. (Didn’t hurt for the HP employee discount on computers….)
I looked at the Heath furniture kits. As a sometimes woodworker, no thanks. My last electronic kit was a Hafler power amp. Need to check it and power it up–it’s been sitting idle for a decade or so. My DIY stuff runs to machine building lately–I have a nice little drum sander for thicknessing and I’d like to use the mirror grinder Real Soon Now.
I think the future of diy hardware is what you are doing, with lots of software in the mix.
Aye! Indeed, much of my “hardware” comes from OpenSCAD source code.
Sort of like VHDL; back in the day, it turned digital hardware design into a software problem, thus combining the worst of both worlds in a single tidy lump. Our Larval Engineer speaks that stuff better than I ever did, not that I ever had much practice. [sigh]