I would not like to look too far into the future, but maybe it would also be worth reflecting on one of the problems. Probably most owners of the E4401 series hope to unlock the 219 option. Due to the measurement method (Y-factor), cooperation with the equipment is required. And here comes the problem under the name E4401-60123. It is not described in CLIP and there is no schematic diagram. I only found one low resolution photo on the internet. The card works with two types of noise sources: traditional sources of the 346 series and newer SNS series (N4000A, N4001A, N4002A). Cooperation with newer ones is more demanding. The noise source has a memory (probably EEPROM) with a stored ENR table and measures its temperature. Working out this can be very difficult. The type 346 sources, on the other hand, only require +28 V voltage switch. A DC/DC converter is placed on the card. One bit is required for ON/OFF keying only. The card itself should work without option 219, there is an option "Press Service, More, Noise Source (On)" in the service menu. And that could be a hook for finding the address of that bit that needs to be switched. Another thing is the card identification. FW should think the card is inserted. This is a way to create hardware that emulates part of the E4401-60123 card to work with 346 series sources. Connectors such as on the E4401 series expansion cards are available from Mouser. The E4401-60123 card itself is available in Keysight as far as I remember. Over $2800...