No, sir. Sahyadri express was a different train. Please refer to the attachments (Source for these Timetables - IRFCA and my own collection)

1) Timetable from April 1971 - There are only 3 trains on the Pune-Miraj route. 2 passengers and the 303/304 Bombay mail. The mail departed from Pune at 10:50 PM and reached Miraj at 11:25AM.
2) Timetables from 1974...
more...

(both UP and DN) - By now there are a lot of trains between Miraj and Pune. These trains are
303/304 Mahalakshmi express - This has the same number as the earlier Bombay mail from the 1971 TT. It also departed from Pune at 10:50 PM like the old mail, but was faster and reached Miraj by 5:35AM. So, the mail was replaced by the Mahalakshmi express
307/308 - Bombay Miraj express - This is the present day Koyna express.
311/312 - Miraj - Pune express. This is the train that was extended to Bombay and renamed to 'Sahyadri express' sometime in 1977.
3) All India timetable from 1977 (Uploaded by Mani Vijay sir on IRFCA) - This is the timetable for the Bombay-Pune route. Note that there is no Sahyadri express listed here. In 1977, it was still running as Pune-Miraj express. It was extended to Bombay by July of 1977 and renamed as 'Sahyadri' express.
There were 2 old trains on the Bombay-Pune route - The mail and the Janata express. The mail was extended to Miraj and later replaced with a faster train named the 'Mahalakshmi express'. The Janata express was sped up, given double decker coaches and renamed as 'Sinhagad' express in 1978.