I got in contact with one of the original developers SCAMP!!
He confirmed, the SCAMP prototype did use a Norelco (Phlips) “carry-corder” (a consumer tape recorder from a few years earlier). This is also mentioned in a 1983 PC Magazine article by Friedl, and that article has a photo of SCAMP opened (the only image of that I’ve ever found). And, this contact said that indeed that “carry-corder” did work but was so unreliable, during demos of the prototype – they’d have to attempt reloading the software many times. So, once it finally loaded correctly, they’d just leave the machine on overnight to be ready for the demo the next day. Keep in mind, that’s the 1973 prototype. The 1975 production used a 3M QIC (DC300), completely different tape system (that 300ft of tape could only store up to 207KB max - but within a few years after 1975, you could get longer tapes that fit in the same package).