My local HFH/Restore had a few cases of Philips F48T12/N Slimlines. As usual with them, they would only sell longer lamps by the case, so I passed.

What color is "Natural?"

If they're vintage lamps, Natural is probably around 3500k, a modern 'Natural' may be around 5000k

GE lists Natural at 3700 K with 90% CRI

Powell

That's a lucky find! 3700K Natural lamps are very expensive and hard to find in non-standard sizes.