Since this thread started Muscleskins have changed their marketing policy.
Every design of swim suit you make has to be available in a range of sizes, colours and prints.
If a shop wants to sell a black 2" classic racing swim brief they might have to hold stock in 7 different sizes, if their minimum reorder were 6 suits in a given size they would have to provide space for 42 pairs. This is quite an investment and explains why we can't get more variety in high street and mall shops.
Muscleskin have gone over to bespoke ordering. You can get any front with any rear provided they are the same width at the side seam. You can from choose a range of fabrics in a range of colours and prints from stock.
They claim "Over 13 Million Combinations!"
I can't find any reference as to whether their products are lined or unlined or whether waist strings can be provided, I suspect not.
Here is their website:
https://www.muscleskins.com/
The Japanese car industry established a production method in which one car was built for every combination of customer options. Whatever option combination the customer chose a car already existed and was shipped to him. If he chose a yellow car with blue seats and carpets a yellow car finished except for it's seats and carpet had blue seats and carpet installed and the car certified as ready for sale. In the factory behind this car a series of part built cars would have one option added and be shunted foward one step in production.
My guess is that Muscleskins keep fronts and rears in different styles, sizes, fabrics and colours in ready bins, when your order comes in in the components are picked and presented to a machinist, the machinist sews the seams and adds the elastic then sends your project to despatch.
With computer aided multi layer fabric cutting available keeping the ready bins full would be easy.