You might think the first baseball themed postage stamp would be issued by the United States, but it was actually issued in the Philippines in 1934. There was a great article in Old Cardboard # 32 from the Spring of 2015 related to these stamps.
After reading that article, I decided to collect these stamps. I used to collect stamps as a kid so I thought it would enjoy these stamps. I was able to pick up all but one of the pre-1950 stamps. The one I haven't gotten yet is the 1935 Colombia issue due to the fact that it is difficult to find as a single stamp and is usually found as part of a 16 stamp set that sells in the $1K range.
Pictured below are the stamps I have so far.
These two are the 1934 Philippines and the 1938 Panama examples.
This is the 1937 Nicaragua complete set of four different colors.
This is the first US Postage stamp with a baseball theme. It is from 1939 and was issued as a centennial of baseball stamp as it was believed to have invented in 1839.
This is my favorite of the different stamps issued before 1950. This is the 1944 Venezuelan complete set of 9 different colors.
This block of four represent a set issued in Japan in 1948 and as you can see one of them includes a baseball scene.
Also issued in 1948, this stamp from Nicaragua shows a close play at the plate.
There are many stamps from around the world issued after 1950 and I have quite a few of them as well. I have these pre-1950 stamps all posted online on my online wantlist website. You can check them out here.
I still need to scan and post most of the stamps from after 1950. I have most of the classics shown already, but there are many more to see.
Enjoy the hobby all...it's an awesome one.