Hello David:
I don't know if you figured out the answer to your question, but coincidentally, I also am building the Scottish Maid.
I know there's much debate on the quality and authenticity of the Artesania Latina kit materials and instructions. On my kit, parts 20 and 21 were as indicated in the "Parts List" as Stern Deck Reinforcements on the same plywood sheet as the bulkheads and false keel, 4mm thick plywood. My research, prior to joining this site, seems to say there have been numerous revisions to the kit and as the errors and omission have been corrected, so has the overall quality.
I found my kit at a flea market in Aberfoyle, Ontario in July of this year. The original owner (not the flea market seller) had started the kit. hence the bulkheads and false keel were missing, but the 4mm plywood sheet was there, allowing me to make replacement parts. I found that if parts are not prefabricated, then you will need to use the various supplied pieces of wood, using the inner box profiles, to match the width, material and length from the "Parts List".
I did sand the stern deck reinforcement parts to match the contour of the false keel and bulkhead, but this was somewhat incorrect. When I started to install the base planks, I realized they must be bent upwards at the stern, so the sanding I did was incorrect. It's impossible to know this in advance with the poor box photos and minimal instructions. I used some Squadron Shop green putty to correct this fault, but I know this is not an accurate remedy. As a novice I accept that there will be many such omissions. I expect to have to improvise often to make the ship esthetically complete. Since I only paid $40 for my kit, I can't really complain. :roll:
My biggest complaint so far with this kit is matching the parts in the clear plastic cases with the often confusing plans. I did not have a detailed list of these prefabricated parts in my kit, if in fact one does exist. I did see a Scottish Maid kit on eBay that did have the prefabricated parts in tiny plastic bags stapled to a cardboard sheet with the appropriate part numbers. I compare this omission to the Billings kits I own that do have a picture and description of the prefabricated parts.
To date, I've completed my hull and added the deck fixtures and masts. I'm realizing that the rigging details are confusing and inaccurate. I'd like to add a more accurate look to the shrouds by fabricating the "serving", but use the kit's ropes.
Regardless, it's a matter of patience and using the resources on this site to help resolve any questions.
Cam