Cannot help with manufacturer, but if you pull the forks out their name maybe stamped on the fork stem.
The Shimano headset should have a date code on it (very small two letters) this (assuming its original) should give you an age bracket, same for the seat post these normally carry a date stamp, along with its size, minimum insertion etc, if these two dates are close then it helps narrow the window, another good site to check through is Velo.
In my humble opinion with the RD cable stop on the top of the chain stay, & the very long rear drop out slots I would guess pre 1980 + -