This is an interesting discussion. I don't know whether you bought this board from us, but we work quite closely with 6 Brothers, so maybe I can help to answer your questions anyway.
Based on your photos and the SKU (model # i.e. HJSJ20B), here are a few comments:
1. It definitely looks like the whole board is shin kaya, but
2. I think that there's a veneer on the playing surface only and I'll explain why they do that below.
3. The SKU you've quoted (HJSJ20B) also indicates a solid shin kaya board, but the SKU is weird.
Firstly the SKU. HJ (hyeongje = brother) SJ (shin bija = shin kaya) 20 (indicates the exact product within the family, thickness, cut etc) B (normally means the board has legs!?).
We don't stock this SKU, but we sell two similar 2.4'' boards; HJSJ21A (single piece shin kaya) and HJSJ23A (multi-piece shin kaya). From your photos, I think the board you have probably is HJSJ21A.
Sometimes 6 Brothers seem to just use whichever display packaging they have left over when packing equipment for us (and I imagine probably for other stores too). Since the packaging is all in Korean anyway, I think they think it doesn't matter for their Western distributors. They use the right packaging when they're selling boards to Korean brick and mortar stores and use the left over stock as just part of the packaging that protects the board.
This has very occasionally led to confusion in the past with expat Korean customers and personally I'd prefer to have display packaging with English printed on it one day, but that's still a long way off being economically viable. Anyway, my point is that whatever it says on the packaging could be accurate, or it could be a red herring.
About the veneer... Usually the single piece shin kaya boards that 6 Brothers sell as HJSJ21A are just a single piece of spruce with the appropriate wood treatments and a playing grid printed on the surface. However, there's a natural variability in the color and grain of spruce (shin kaya). Because of that, about 10-20% of boards made in this style are deemed too dark, or the grain too contrasting for the surface to be comfortable to play on.
In these cases, they use a lighter piece of spruce to create a veneer for the playing surface only, because they think this makes it easier on the eyes when playing. This technique is regarded as preferable to painting the surface (like they do with agathis, which is again to make it easier on the eyes). The whole board is still shin kaya, but it looks to me like yours does have a veneer treatment on the playing surface and it actually requires a very high degree of skill to do it this well.
In Korea, these boards are apparently just sold for the same price alongside the other 80%+ which are made from lighter colored spruce and nobody minds. We thought that Western Go players might not like this style of board, so we asked 6 Brothers to only select boards from the 'other 80%' which don't have this treatment when shipping to Go Game Guru.
If you ordered this board from us, then someone must have made a mistake at the factory and I'd appreciate it if you could let me know please. And, if you did order the board from us and you don't like it, then we'd be happy to exchange it for you.
You didn't post a photo of the bottom of the board in the three above, but one way to confirm my explanation for what you're seeing is to compare the color and grain on the top and the bottom of the board. Is the bottom of the board a much darker color? If so, it supports what I've written above.
Either way, it's a nice Go board you have there and I hope this explanation helps to solve the mystery for you

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