Never seen anything remotely like a dimple on the struts top.
I had a few issues though with aftermarket strut inserts (which were slightly wider).
They didn't properly fit due to the struts tube being deformed slightly where the bracket for the brake hose was welded on.
If the lock nut doesn't move, that's because it is rusted solid to the strut.
So you'll need penetrating oil and patience, or heat.
But the last option will ruin the insert.
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<font color="blue"><i>1980 TR7 DHC (my first car, now restored and back on the road)
1981 TR7 FHC Sprint (better known as 't Kreng)</font id="blue">
<b>[url="http://www.tr7beans.blogspot.com/"]<u><b><font size="2"><font color="red">My Weblog</font id="red"></font id="size2"></b></u>[/url]</b></i></center>