T-Mar

Thank-you for the additional information and pictures. The presence of down tube water bottle bosses also support 1984 manufacture. While these and the brake cable tunnels could have been added by a previous owner, it seems unlikely given the short dropouts, which are consistent with the era. I'm not aware of SunTour having manufactured hangerless dropouts, so it would appear that the dropout hanger has been removed. However, I doubt that it ever had brazed-on shift lever bosses, given the presence of the clamp stop.

A 27.2mm post suggests an Ishiwata tubeset, as both Tange and Miyata (who drew their own butted tubes) would use a post no larger than 26.8mm useless it was Tange Prestige, in which case I would definitely expect brazed-on shift lever bosses and better overall workmanship standards. The major Ishiwata users during this era were Bianchi, Bridgestone, Fuji, Kabuki, Maruishi, Ross and Schwinn. The serial number format does not fit Bianchi, Fuji, Maruishi or Ross. Both Kabuki and Schwinn were contract manufactured by Bridgestone, so it does appear to be at least Bridgestone manufacture, if not a real Bridgestone.

A 27.2mm seat post is quite large. Ishiwata 022 used this size of post but the lack of brazed-on shift levers bosses would be atypical for an 022 frame of this era. More likely, it is seamed Ishiwata EX-F, possibly with lower grade Magny (carbon manganese) or hi-tensile forks and stays.

My leading candidate would be Bridgestone 400. The frame was an unspecified triple butted CrMo main triangle and this would be consistent with Ishiwata EX. The stays and forks were hi-tensile steel. The frame features are also consistent for a Bridgestone 400, with the exception of a derailleur hanger and dropout eyelets, which as discussed earlier, were probably removed. Even the steel headset, which is the component most likely to be OEM, is correct.