I've heard that the heads were sourced from Otari a couple of times but without any sources to back up the claims. I just shot off an email to Otari technical service to see if I can find out for sure. I also asked if Technics may have sourced other tape specific items (pinch rollers for instance) from them or if they could help with the OEM supplier (I have an email off to Technics too). As far as the heads go, I don't know if it's such a great idea to replace them on either machine with the ones they originally came with. Aside from Greg's extended response heads (which would be the first choice) which will certainly have better sonics and wear characteristics, JFR Magnetics can replace the heads on either machine with better ones for a more than reasonable price. John also goes through the head assembly and makes sure that every thing's up to snuff. If I recall correctly Mikey P. had John install new heads (1/2 & 1/4 track,..one was a Panasonic and one was a Tascam) in his 1500 and it was around $650 with installation and servicing.
BTW One of the times that I saw a posting (over on Tape Trail) suggesting that the two machines had the same heads, was in reference to soft, fast wearing heads. As I understand it, the softer the alloy, the more permeable the head and the more sensitive it is to small flux differences. But, the softer the alloy, the more quickly the head wears out. That would seem to be 'one' of the balancing acts involved in head design.
Personally, getting the best (Flux Magnetics) is cheap enough (in terms of this hobby at least) that I really never debated the point. Or maybe I did when I was young and stupid,.. like this time last year!