The seven Raytheon Lectron System Add-On kits allowed the Series 1 – 3 sets to be upgraded so that additional experiments could be conducted without having to purchase a brand new model as an upgrade. For example, purchasing Add-On kits 1, 2, and 3 would effectively make a Series 1 kit into a Series 2 kit.

The Add-On kits were released in concert with the sale of the refreshed Raytheon Series 1 – 5 models in May of 1968.

The Add-On kits were released in two different sized boxes, dictated by which blocks were in the kit.

Add-On kits #6 and #7 included their own set of experiment instructions.

Although one document alluded to the existence of an 8th Add-On kit, it was never productized and what blocks were to be  included remains unknown.

From this document however, we do have a Mr. Harley Bates identified as the ‘keeper’ of new experiments that might have led to a supplementary manual no. 2.