The console looks like some sort of molded SIDE console, and thus not an OEM console, unless that console is on center and then it could be a CENTER CONSOLE 15. It is hard to tell the location of the console from the photograph. Since the arrangement of the interior defines what model the 15-foot hull was called, a photograph with better illumination that clearly shows the interior layout will be necessary to determine if that console is OEM or aftermarket.
The Boston Whaler 15-foot hull is only 15-feet 3-inches long. Your measurement of "15.6-feet" translates to 15-feet 7-inches, too long for the 15-foot hull. Check your measurement.
The thwart seat with seat back is also likely not OEM.
You can easily get more information if you find the STENCIL NUMBER and HIN. See the FAQ for advice on STENCIL NUMBER and HIN.
If the STENCIL NUMBER and HIN identify the boat as a 1978 model year, then the boat was originally a SPORT 15, as that was the only model offered in 1978. See the 15-foot hull article linked below.
FAQ Q1: How Old is My Boston Whaler?http://continuouswave.com/whaler/reference/FAQ/#Q1The HIN will positively identify the production epoch. The STENCIL NUMBER will positively identify the model series.
For more about the 15-foot hull and various models, see the REFERENCE section article on the 15-foot hull:
The 15-foot Hullhttp://continuouswave.com/whaler/reference/15/[Nota bene: do not post images that contain large black borders and use portrait framing of objects that are better photographed in landscape mode. Badly framed images with large black areas are not useful and are deleted.]