This firm produced perfins from 1914 to 1938.
I am sorry to report that no perfin covers on company letterhead have been located or reported. However, as a consolation prize I will show you what to look for. The example is an envelope used 28 May 1918 from the same approximate time period that the company was putting perfined stamps on their envelopes.
National Paper & Type Co. cover without perfin
There are four known NPT perfin covers, all probably incidental or unauthorized usages. They are:
National Paper & Type Co. perfin on cover to Los Angeles, CA, April 1938. The stated return address, Apartado 666 (P.O. Box 666),
is the correct box number of National Paper & Type Company according to several Google book "hits".
It confirms that National Paper & Type Co. is the pattern owner.
National Paper & Type Co. perfin on picture postcard to Brazil, 4 May 1927.
National Paper & Type Co. perfin on picture postcard to Brazil, 13 September 1927.
National Paper & Type Co. perfin on picture postcard to New Zealand, 3 December 1927.
And here are some of the stamps which have NPT perfins: