Yes, Sylvania, later Philips-ECG. Made in the same factory on the same tooling by the same people, Philips-ECG production is USUALLY identified with blue screened labelling and green for Sylvania, but I think I've seen green Philips labelled tubes. Certainly I've seen blue ONLY on Philips production 6L6GCs. Earlier Sylvanias have yellow printing, but I've never seen yellow print Sylvanias that were STRs. I think that they switched to green at a certain point in time.
In all cases, the print can be on the base or on the glass. It tends to stay put on the base, but often flakes off the glass or is easily wiped off.
Be warned, at the very last, some garbage Chinese made tubes were labelled with the Philips label. They are very different tubes, the differences are obvious from quite a distance away, and they are strictly to be avoided. They are notorious for shorting out and damaging output transformers.
I'm kind of a nut for the STRs. To me, any Mark III or earlier should have STR 415s in it, or at least 387s. They were designed to run with those tubes, and that's what they run best on.