Your piece is a Higgins & Gage # B-23.
Most of these items were released in the late 50's to early 60's.
In the mid 1960's the Indian PO delegated these to general use as postage and were used up by the various satellite regional PO's by overpriniting the envelopes and covering up the Express mail emblems. The "Locals" can be ID'ed by the method of blocking the "express mail label and the overprint on the stamped value .. Here are 2 examples...

The top one is from Madhya Pradesh PO and the bottom one is from the Maharashtra PO.
There are 20-30 different varieties of these envelopes, and are a insteresting side collection.
Do a Inet search for Express Delivery Cover & Stationery. You may be interested in the results.