It looks exactly like mine, which was also very tight when I first removed it. I wrapped the threaded portion in some thin rubber and held it in mole grips. The top knob is simply screwed on to the threaded portion and if it has been screwed down too far will be tight and also cover the grubscrews or it may have been installed with the grubscrews at the top of the threaded portion. If so, you have to get the knob off first.. Try wrapping the knob with some thin rubber as well and give it another go. Some mechanical grips on the knob as well might help or maybe a spray of wd40 up inside the knob.
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Doug
2011 Plus 4 in Rich Maroon

1972 750 “ComDom” sprinter
1958 Triton 650
1992 Triumph Trophy 900