PDP-11 tape question

[John Forecast]

On Jun 23, 2020, at 9:57 PM, Chuck Guzis via cctalk <cctalk at classiccmp.org> wrote:
> 
> I've been processing some PDP-11 9 track (800 NRZI) tapes and run across
> something that I don't recognize.
> 
> Every file on the tape consists of a number of 512 byte blocks (okay,
> that's normal) but at the head of each file, there's a short block of 14
> bytes.
> 
> Usually, a short record like this is discarded as "noise" on many
> mainframe tape systems, but here it's consistently present. Here's what
> one of the records looks like:
> 
> 15 34 fe 51 fe 76 01 01 00 00 01 80 10 00
> 
> Doesn't seem like a file name in RAD50 format, so I'm puzzled.
> 
> Inquiring minds want to know...
> 
> Thanks,
> Chuck

Looks like a standard DOS/BATCH-11 tape file header. The first 6 bytes and the last 2 bytes are the file name in rad50 - “HMMMD0P.SAV”.
Depending on where/when it was written the header can be either 12 or 14 bytes:

Format of DOS-11 magnetic tape header records:

	.rad50	/fn1fn2/	;1st 6 letters of filename
	.rad50	/ext/		;3-letter extension
	.byte	proj,prog	;ppn (display it in octal)
	.word	prot		;protection code, usually <233> octal
	.word	date		;date: (year-1970.)*1000.+(day within year)
	.rad50	/fn3/		;[optional] letters 7::9 of filename

Length=14. bytes (or 12. if no fn3, like in actual DOS/BATCH)

  John.