Square block drain

[LazyK]

 to achieve the flush or recessed  head plugs  are NPS National Pipe straight Tapping in the block is NPT

from the link posted by "sniper" above

For rigidity; sealable only with sealant; sometimes male straight enclosed by female tapered for low-pressure sealing

[NoGoFargo]

13 minutes ago, LazyK said:

 to achieve the flush or recessed  head plugs  are NPS National Pipe straight Tapping in the block is NPT

from the link posted by "sniper" above

For rigidity; sealable only with sealant; sometimes male straight enclosed by female tapered for low-pressure sealing

As a Plumber I have to say mixing NPT with NPS is a very risky business even at very low pressure. Almost guaranteed to leak at some point and will stretch the NPT threads. 
 

There are an abundance of NPT “ Socket Head Pipe Plugs “ designed for flush installation available. 
 

https://www.fastenal.ca/mb/productList/Pneumatics/Pneumatic+Fittings/Tube+Fittings/Socket+Pipe+Plugs/604269?pageSize=24

[Bryan]

It might be just the problem is me eyeballing it. The book says the plug is tapered and that's what I bought.