As far as I know the EL35 is the Octal version of the EL5 which has a side-contact base.
Philips introduced the EL5 in May 1936, while RCA introduced the 6L6 in April 1936. So I don't think the EL5 and the EL35 were based on the 6L6. The characteristics of the EL5 and 6L6 are close indeed but I personally would not call them electrically equivalent.
Later many variants of the EL5 were introduced. Some had the anode lead coming out at a top cap, so could be run at much higher anode voltages. See:
https://frank.pocnet.net/short/054/2/224.pdf
When I just rediscovered this hobby I breadboarded a couple of amplifiers using the EL5. In push-pull ultra linear with cathode feedback they sounded very nice to my ears/brain, even though I was running them rather 'cold'.