Hi Eddie,
Apologies if I may have previously given you incorrect information. I looked into heaters a while ago and from studying lots of pictures, found that there were two different designs of heater box, one incorporated a fan and was more rounded at one end and the other was the rectangular box shape as you have in your Aceca, this is also the same in mine (BE786). The rectangular box type, which I believe is the same as a Frog Eye Sprite, seemed to be fitted on the later Aceca models, possibly around 1959 onwards (VERY approximate so please don't rely on this info).
In amongst the many boxes of bits which came with my car, along with all the other heater parts was a Smiths heater fan motor complete with mounting brackets etc and I assumed (possibly incorrectly) that this was a standard fitment to go with the rectangular heater box type.
I'm not really going to know whether this was factory fitted or added as a later modification until I get to the reassembly stage which is still a little way off, but I'll update this post with any info which may help anyone in the future.
Kind regards
Nick