It was some sort of stunt mechanism. It was only there for the sparks so the actual board shouldn't have it.
Also I think that at least a few boards were plywood. You can see the layers in this picture.
http://cursoryviews.files.wordpress.com/2009/07/back-to-the-future.jpg
yes at first glance i thought it was cedar. like barn board cedar. the first page has a bunch of great reference pics, and after you said you used plywood i took a greater look. sure enough it is plywood!! which i don't believe was around in 1955

so I'm with you 100% that they used plywood. i have also been watching eBay for skates to my surprise some that look pretty close are very inexpensive. there are 2 or 3 pairs that have the same 2 groves in the bottom of the skate like the picture on the first page once i get back into the country I'm going to go hardcore and watch see if i can grab a pair
i also was watching earlier today on youtube how to weather wood. the picture you posted looks like you can achieve that blue/dirty look with some vinegar and steel wool. i also think the "tail" part of the board is external cedar trim board you
heres a example
the second one in from the left. though it is much smaller, older homes have these up to 5" wide. you can also see the matching cedar grain from the pics on page one
as for the sparking device, i always thought they used something like a cutter torch ignitor or something under neath.
if you removed the thing in the center part of the dish and mounted them under neath i think you can achieve the same effect, thats how i always thought they did it until now where you can see some kind of metal there? I'm curious how that works lol
im super addicted to think now having the measurements, i look forward to you finishing your build!