That HP site, Sphere I think as the other poster mentioned, is a good site to check.
The other thing you might consider doing is looking in some of the old HP schematics to see if you find them. A lot of HP parts lists had the complete part number on them where the part itself was missing the first two digits. One other thing you can do is make up a small amplifier not unlike the 2n5179 HF amp and run a signal generator thru it to find the upper frequency limit. That's the issue with the ECG123 vs the 123A, etc. where they use it as a general purpose small signal replacement.