In reference to the patent: US1594978 I'd like to know, if the such patent is still covered by the copyright laws and, then, if its replicability is still allowed for the development of similar products (i.e. a new pen for a different market). In addition, I also would like to know if a pen based on this patent can be be sumbitted for an original patent grant simply citing the one mentioned. If I remember good, the validity of patents ranges from 10 to 20 years. Any suggestion or clue is really welcomed.
I find your question a bit confusing. Patents and copyrights are different types of intellectual property. Asking if a patent is still covered by copyright makes no sense. Patents protect inventions. In return for making public how the invention works, you get a limited period of exclusive use (typically 20 years from the filing date). You need to apply for patents and they need to be approved by the government. Copyrights protect original works like books, recordings, art and films. You don't need to apply for copyrights although you can register them. I encourage you to review the articles in Wikipedia about patents and copyrights to learn more.
The cited patent patent probably expired in 1943. You can safely make and sell the exact pencil described in it. Not only in a different market, but even the same market. What you can't do is submit a new patent based on it. Once the technology described in a patent expires, it becomes available to everyone and anyone. You can't own it. If you make an improvement to the device and that improvement is itself useful and novel, you may be able to patent the improvement. You should however make yourself aware of other relevant patents when designing your own product to make sure there aren't other patents still in effect that must be avoided.