Thanks mateThey are different sizes. The E40 bulb will work fine on E39 sockets, but E39 bulbs won't always work perfectly on E40 sockets. I changed my E40 socket on my Giesemann Spectra and now I use either types of bulbs (E39 or E40) with the E39 socket. Please contact Dave at Hamilton. That's where I've got my E39 from and it's relatively easy to change them.