Hey Guys, Have been doing a little more digging and it looks like the 86, 87, 88 CR500 forks were made like this by Honda i.e. dropping in size as you go up the fork, 43mm botttom yoke and 41mm top. here's a pic of an 87 and you can clearly see the line where it changes thye same as mine....
The 86 - 88 ones are cartridge forks and from what I've read the XR600 ones were based on and pretty much the same as these, but, without the size difference in stem diameter. Going by this bit of blurb I found trawling and more photo's I've found the CR250 ones seem to be the same from 86 - 87, not sure if there was a length difference though. Apparently the axle size is 17mm, but, I will check this with mine...
"By 1987, the conventional forks were developed to a very high level and
most agree the 87 cartridge forks on the CR250 and 500 were some of the
best ever offered on any production bike. In 1988, Honda switched to the
upside down forks (89 for the 500) and it was all down hill for many years.
It's the same as the debate about Steel vs. Aluminum frames, not everyone
is convinced the switch to Aluminum was a good idea. In fact in 97, if I
remember correctly, Suzuki switched back to conventional forks on the RMs."
It seems it might give us another option for upgrades and I will pull my finger out and measure that axle