An update.
Kinda longwinded so please bear with me.
After a bunch of research & knockin heads with other DB fanatics I've come to this conclusion,hopefully it's correct.
I've compared this frame to all of the other DB models,it's exactly like my 79 Sr Pro & 80 Med Pro except for one thing.The headtube angle is off,otherwise it's built as a SR Pro.The headtube angle is a match for my 78 frame,the 79 & 80 are slightly more angled.
This is a 78/79 built Proto of a 79 Sr Pro.
This pic is of the closest matches,the others were left out so as not to confuse.
The chrome is the 79 Sr.
The blue is the 80 Med.
The red is the 78.
And the black is the Proto.
Now comes the interesting stuff.
Here's a pic of the Proto & the 78,notice how the headtubes are a perfect match.
Sorry but it was the best pic I could take,being that close distorts the view but it's exact.
Here's the Proto with the 80,lesson learned from pic 1.
Here's the Proto with the 79,it's closer but still not exact.
If the cups were removed it'd match with the 80.
All the other geometry matches up with all 3 frames,but here's another difference.
Notice the size of the diamond cut out & the arc cut of the gusset.The arc is the slighest of any DB out there,none of my other 9 frames each of a different model has that same slight arc to it,it's more pronounced like on every other DB I've seen.
Now the diamond cut out is larger by far on the Proto than the red 78,the other DB's I have is closer in size but still smaller(Yes I measured).These gusset plates were stamped not cut by hand,so that theory is out the window.
Here's better shots of the geometry,they're not perfectly aligned but you get the idea.
Also take a look at where the down tube is attached to the bb on the red 78 compared to any other DB out there including the pics above.It's more centered on the 78 than any other DB,every other DB has the connection near the bottom of the bb.
Including the Proto which can be seen below.
And lastly,every 79/83 DB has a serial similar to this.
This is the serial on the Proto,I had to take a close up so it could be seen on the black.
Ok I'm done now,gonna go rest my typing hands.