So far we have been using springs with 11 mm height, made in ss304 stainless steel, 0.5mm thickness, with the right size for the HP41c and woodstock battery cases. However, the height and spring effect proved to be too much in many cases: the pressure required to open the machines was too much. Also we had to sell them in pairs for the HP41, since very often our spring was taller than the one to be replaced.
In the picture besides you can see the old spring (at the back) and the new spring (at the front)
So we have taken off a turn in the spring and reduced the height to 8mm. It still works well but it forces less the calculator - in particular the fragile upper posts of the calculator.
By default we will serve these lower height springs, but we still have some longer springs if you abosolutely need them.
We will equip the battery holders with these too - it will make your calculator last longer. In the case of the Woodstock, this is very apparent - the previous ones were a quite tight fit.