Base variant is unlocked after node in progression
Most block variant groups have the first unlocked block as the first block in the group. Therefore, when you unlock the first block, after you build it the first time, the other blocks are available by placing it on your toolbar and using the scollwheel.
However, a few blocks have the first or "base" block later in progression. This means that you have to add a block to your toolbar, build that block, then replace it on your toolbar with the base variety. A couple of examples are the Basic Refinery (Refinery is the first variant block listed) and the Beacon (which is unlocked first, but the Antenna is the first variant block).
These are very simple SBC changes, but for people who run through progression often, this saves a lot of time placing things on your toolbar only to replace them a moment later after you build the block.