I own Serial #253, I was told that Bob started the numbering system with #100 ? So if thats true mine is the 153 Taylor.
The 815 is a 1976. 38 years old now and sounds amazing. Is in excelent condition for it's age ,and this 815 was very well constructed. I know there are some American Dreams Guitars out there, Please speak up 
um, i did a little more research & according to the serial #, your 815 could be a 77 model 
this used to be on the previous iteration of Taylor's website
http://www.themusiczoo.com/blog/2010/taylor-guitars-serial-number-dating-guide/SERIAL NUMBERS YEAR SIGNIFICANT DEVELOPMENTS
[didn't use serial numbers!] 1974 Employees Bob Taylor, Kurt Listug, and Steve Schemmer purchase the
American Dream Musical Instrument Manufacturing shop in Lemon Grove,
California, and change the name to Taylor Guitars; all rosewood used is Brazilian.
10109 to 10146 1975 Had to let most of our employees go after we used up our original capital;
settled into being broke and trying to figure out how to make guitars.
20147 to 20315 1976 Guitar #20168 is the first Taylor made from East Indian rosewood; guitar
#20179 is the latest Taylor “on record” to have the mortise neck joint
(by the time we made #20229, we were not using mortise neck joints);
Kurt hits the road to sell direct to dealers.
30316 to [--]450 1977 Taylor Guitars signs a distribution deal with the Rothschild company,
an instrument distributor.
there shows to be 37 made in 74 & 168 made in 75, if the sequence was continuous with all numbers were
used & the year was when the guitar was started, if i recall correctly, the idea for starting with #10109
was that #10101- 10108 were reserved as numbers #1-8, so that those guitars could built "later on" -
depending on when the sequence changed from 30316 to a 3 digit number, if it started at one or continued, say for
example if there were 14 models made, to 30330 & then the transition was made, the first 3 digit serial could be #220
but regardless, your 815 has an earlier serial # than my 710, so you're still "in the lead"
so far,
11 814ce
03 312ceLTD
99 BT
95 DCSM
91 412
89 810c
87 510
83 615
77 710
77 815