Why is RSA encryption version 1.0 not specified in an RFC? I saw that there are three RSA encryption versions, as follows:


RSA encryption version 1.5 (RFC 2313)
RSA cryptography specification version 2.0 (RFC 2347)
RSA cryptography specification version 2.1 (RFC 3447)


Why is there no RFC for RSA version 1.0?
RSA is and was specified by the PKCS#1 specifications of RSA laboratories. PKCS are the "Public Key Cryptography Standards" by RSA Laboratories, now part of EMC2. The RSA PKCS#1 v1.5 is the lowest publicly released version of RSA by RSA labs that can currently be downloaded.

Version 1.0 to 1.4 are working drafts as specified in the PKCS#1 documents themselves and at least the inital PKCS#1 RFC 2313 as well:


Versions 1.0–1.3 were distributed to participants in RSA Data Security, Inc.'s Public-Key Cryptography Standards meetings in February and March 1991.
Version 1.4 was part of the June 3, 1991 initial public release of PKCS. Version 1.4 was published as NIST/OSI Implementors' Workshop document SEC-SIG-91-18.


Although the RSA standards have been defined by RSA Laboratories they are likely to have been influenced by the participants to the meetings and workshops.

The RFC's are simply a reiteration of the PKCS#1 standards that have undergone public scrutiny. They are not in control by a single private party. So the standard may be identical, but it is at least open. In general the RFC's just keep to the PKCS#1 specifications, but at least in principle follow up documents may deviate from the specifications by RSA.