Ken

I don't know what pre-OSHA is, but the trailer mounted generator or pump in the foreground might give you a clue.  My feeling is that the trailer is unlikely to be much older than late 1960s or early 1970s.

As far as ammonium bifluoride goes, we have trialled its use on sandstone buildings here in Australia on several occasions, and recently I used it on a sandstone monument.  It appears to work in a similar fashion to hydrofluoric acid, but is much less aggressive.

We were not able to detect any significant damage to the sandstone as a result of using it.  I suspect that it might be a problem if used on a quartzite rather than an argillaceous sandstone (where it might etch the quartz if used in a high concentration), and I do not know what would happen if you had a calcareous sandstone to deal with.

Concentrations of up to about 10% w/v of ammonium bifluoride seem to be pretty innocuous, but above that, the stone can sometimes become a little bleached.

Anyway, it is not always effective.

Hope this helps.

david