Garry,
here is a slightly different version of the same. When I bought my car, it came with the armrest. I put it aside and bought the original piece to have it, together with other woods brought back to their original shine. I kept the armrest however and may use it as option on longer trips such as going in August to the European event
You can see from the pictures, that my armrest is slightly shorter that Baron's. His starts immediately after the ashtray, while mine has a small space between the ashtray and actual armrest. Question of choice I guess.
In the case of my armrest the top slides, while it would make more sense if it could have a hint and open up, which is I believe easier to do and does not require greasing the sliding path, which I have to do quite often
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Concerning measures, I think that you do not need that from us as they are very intuitive. Basically, for horizontal measures, the modification it track the dimensions of the original box. In the case of my item, it was actually build over the original box then the new veneer was applied all over the stuff. As for the hight must be decided by your comfort position. In my case, the hight is 26 cm including the pad, with the pad itself being 3 cm thick.
hope it helps.
BTW, if you need a new OEM ashtray, I have a local guys that makes them and supplies to resellers. Not cheap. but he can insert the leather of your choice that tracks your desired color of the interior:-)