After the early 1880s, when series fountain pen production started, all companies were using hard rubber. Initially black hard rubber was used but soon companies like Mabie Todd started using red and black mottled hard rubber. After the turn of the century Parker and Conklin experimented with green woodgrain and soon most companies were using red and black woodgrain. After the First World War, Watermans refined the woodgrain design' into a ripple pattern and used that pattern into the thirties.
During the twenties, fountain pen companies (initially Parker, Watermans and Sheaffer) started using bright red hard rubber to see what the market could stand and when they found plastics to be more versatile than hard rubber, they made pens initially in red plastics
With the introduction of plastics, profusions of colours appeared, and combinations of colours became common, particularly black and pearl initially, followed by numerous different designs including striations .
At first, the major companies ordered all the production run of a certain type of plastic so that they could have exclusivity in the colour. Twenties pens therefore can be identified with some degree of certainty as to the manufacturer by merely looking at the type of plastic used.
The position became complicated in the thirties when the depression and concomitant attempts to stem losses decreed that the pen companies couldn't easily finance holding of large stocks of plastics which they needed to manufacture pens.
This capital expenditure problem was compounded by the fact that after manufacture of the rod stock, the plastic needed to wait a considerable time to cure before it could be drilled out to make the caps and barrels. Although the companies ordered the rod stock to be made for them as and when they needed it, very few companies wanted to finance such stocks, especially if they couldn't utilise the rods immediately.
A co-operative was thus formed to order and hold the stocks out of which the companies bought the plastic as and when they needed it.
The result of this is that with the exception of (primarily ) Sheaffer, who used their own rod stock and developed their own techniques for stabilising it quickly after manufacture, often plastic materials can be found coming from a plethora of different manufacturers.
All the plastics used in the expensive Watermans Patrician can be found in relatively inexpensive depression Parkers, and even Leboeuf used plastics which were common to such companies manufacturing very low-end pens like Morrison.
Although this situation did not apply to the materials which were made up from compressed layered sheeting, such as the Parker material used in Vacumatics, the commonality was not confined to american pens. The supposedly exclusive Bayer Celluloid much touted by Montblanc in the thirties for their rare and desirable Platinum Lined (PL) series can also be found in pens like the very high-end Omas Extra Lucens of the same period.