Proof of concept or proof-of-concept, noun or adjective

Solution 1:

Nouns are often used like adjectives. When used in this fashion, they are called noun adjuncts or attributive nouns.

For example

• chicken soup
• horse barn
• baseball diamond

Proof of concept is either a noun (when written with dashes) or a noun phrase, either of which can be used adjectivally.

Solution 2:

Generally, It is correct to say

My proof-of-concept implementation

since you can have a "proof-of-concept implementation" and a "final implementation"

it is wrong to say

My proof-of-concept prototype

since you don't have "proof-of-concept prototype" and a "final prototype"

Unless your prototypes cost millions of dollars for each one. e.g. aerospace. Then you can have var. stages of the prototype itself. Otherwise, the prototype is a stage of a bigger process