Is This Thick Guideline On A 394 Pair OK?

What is the perforation? Your pair is definitely not Perforated 12. Based on the scan it looks like it is perforated 8 1/2. If so it is probably genuine if it has a single line watermark. There were no 3¢ imperforates with a single line watermark. The line is a different matter as it is almost assuredly fake.

perf 8.5 yes

I decided to examine the watermark more closely. It certainly looks like a single line wm at first glance. But now that I'm staring at it, I'm talking myself into wondering whether it is legit or not. Here is a photo in fluid. Anyone want to chime in?

the ink for the guidline shows a halos around many of the perfs. IS that common? darker around the edges of each perf.

That's been pointed out earlier in the thread. It is a good clue that the guideline is fake.

thanks Risny, I missed the first page.

It appears to be a genuine single line watermark. The watermark 191 in my experience are easier and clearer to detect. The single lines are the bane of my existence (sometimes)in this series the WFs.
A quick review of Armstrongs book indicated that fraudulent watermarks are rare and made by impression or embossing the stamp with the suitable punch. Noticeable on the front or reverse as a inset. sHe further stated regum to hide the strike is common within this rarity.I do not think that applies here.
Often when one finds something wrong with one area of a stamp it causes concern as to what else can be wrong. I think that does apply here, which has led to the vigorous thread. Interesting discussion.
Cheers, mark

Edit for correction: Armstong does not mention regumming, I don't know where I read that, possibly one of the Schmidt books.