I got a cover of When I first started collecting stamps

Last month, I've been organizing Japanese stamps, during the period I found some covers used in my junior hood when I started collecting stamps.
Shown below is an example of usage franking the commemoratives issued in 1977. It is not a single franking, the envelope seems to be rather big, so I do not want to store it before, however it turns out to be not common these days and my philatelic friend also admit it.

As a general collector, I have completed all the Japanese commemoratives, so I decided to make a specialized collection of the commemoratives of which postal rate is 50 yen.(1976-1979)  Any Japanese commemoratives are not classic stamps and I collet mainly classic stamps such as German States and Switzerland, so I'd like to collect super modern stamps this time and I don't think there are many competitor.

Block of four, New-National-Treasure series Vol.2 50 yen with a pair of 75 yen defenitive =350 yen, a easy-recorded letter addressed to Shizuoka Prefecture

Kashiwa post office, April 16th 1978,  comb-shaped cancellation.