I had an off the record conversation with a rep at ITW insulation systems. Dow doesn't make this stuff anymore. Period. They have franchised it out to hundreds of manufacturers across the country and around the world. There are going to be variations from every one of them. Here is what I know for sure.
What used to be called FB stood for Fabrication Billet. Not flotation billet. It is now officially called XPS PIB as sold in the big blocks.
http://www.itwinsulation.com/Trymer/library/Data_Sheets/ITW_XPS_new_Data_Sheet.pdf
Flotation billet or Dock Billet is now officially called Buoyancy Billet. Kent posted the link in the above post.
FB and BB are NOT the same thing.
If you want a high compression strength of 40 PSI you are now talking about Building products. Highload and Roofmate. Most are over 2pcf.
http://dow-styrofoam.custhelp.com/app/answers/detail/a_id/815/kw/compression%20strength
http://dow-styrofoam.custhelp.com/app/answers/detail/a_id/665/kw/compression%20strength/related/1
XPS PIB is closest to what is called for in the plans with regard to density, cell size, and compression strength. It is available all over the place relatively cheap.
I understand there has been some testing regarding peel strength with respect to cell size and large cell is greatly preferred. However, due to variations in the extruding process and the newest EPA certified blowing agents and methods, there is no guarantee that any particular type of Styrofoam is going to have the cell size you are looking for but is most closely associated with XPS PIB. It just isn't a value that manufacturers control for. There is some luck involved. You need to look at a sample before you buy.
ACS and Wick's could have a supplier or manufacturer that is trying to supply what our plans call for. I wouldn't bet that they would tell anyone who the supplier is.
There isn't any source I can find that would have a product with the guaranteed specifications of 1.6 pcf, 40psi, large cell size.
Additionally, the density of the product determines the compression strength. According to Dow's tables,
1.55 PCF has a compression strength of ~20 psi,
1.80 PCF has a compression strength of ~40 psi
2.20 PCF has a compression strength of ~60 psi.
http://dow-styrofoam.custhelp.com/app/answers/detail/a_id/848/kw/dock/related/1
ITW is the largest supplier.
http://www.itwinsulation.com/wheretobuy/wheretobuy.asp