John McVie Strings

According to this fleabay ad, maybe it was Guild? Prob one of many endorsements.

“Guild electric bass strings un-
cover the bass player. You can
hear what you’re doing, and get
into the feel of the music. They can
hear what you’re doing—Guild
strings make that bass drive and
support come through strong.
Guild Round Bass strings de-
liver solid punch in any of four
gauges. They last long, and they make sure
your frets do. too. Guild Flat Bass, also in
four gauges, are smooth on the fingers and
on the ears. With more windings than other
flat wounds. And new Guild
Ground Bass combine the sound
of round wounds with the feel of
flat wounds. All are constructed
to give you the full spectrum of
highs and lows on each note, for
tone, clarity, depth and sustain.
That's why great bass players
like John McVie of Fleetwood
Mac use Guild bass strings. Guilds help you
make your presence felt—and heard. Buy a
set for your bass; the improvement will be
clear. And that’s the bottom line.”

Luckydog said:

According to this fleabay ad, maybe it was Guild? Prob one of many endorsements.

“Guild electric bass strings un-
cover the bass player. You can
hear what you’re doing, and get
into the feel of the music. They can
hear what you’re doing—Guild
strings make that bass drive and
support come through strong.
Guild Round Bass strings de-
liver solid punch in any of four
gauges. They last long, and they make sure
your frets do. too. Guild Flat Bass, also in
four gauges, are smooth on the fingers and
on the ears. With more windings than other
flat wounds. And new Guild
Ground Bass combine the sound
of round wounds with the feel of
flat wounds. All are constructed
to give you the full spectrum of
highs and lows on each note, for
tone, clarity, depth and sustain.
That's why great bass players
like John McVie of Fleetwood
Mac use Guild bass strings. Guilds help you
make your presence felt—and heard. Buy a
set for your bass; the improvement will be
clear. And that’s the bottom line.”
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1979 Fleetwood Mac John McVie Guild B301 Bass Guitar Strings - Vintage Ad | eBay
1979 GUILD BASS STRINGS "Let Out Your Bass Instincts" Magazine Ad | eBay
Let out your bass instincts - Guild advertisement (1978) >> Vintage Guitar and Bass

Very cool! Thanks for this!

Per this scan from Ernie Ball literature of about the same era (provided by @rick harper in this invaluable post), also covering their contemporary competition,

the four flatwound Guild sets (bottom right of scan) came in the same gauges as Ernie Ball's "groups" (top left of scan; a classification EB still uses for their traditional flatwound sets).

In other words, the Guild F-400H were 55-75-90-110; the F-400M were 50-70-85-105; the F-400L were 45-65-80-100; the F-400XL were 40-60-70-95. No idea which ones McVie used, nor whether or not he already used them during the Fleetwood Mac (1975) or Rumours (1977, rec. 1976) sessions.

I think he had quite an arsenal of basses, from what I understand the Rumors album was mostly an Alembic series I with flatwounds, maybe Guild strings , maybe something else, who knows

And in those days, Guild used D'Addario strings, as did Ernie Ball. The Guild information is from our Guild inside sales rep.

EB did not own a string machine, they were D'Addario's west coast warehouse at the time, and they used the same 50/70/85/105 and 45/65/80/100 when almost everyone else used 45/65/85/105. D'Addario used increments of 20 between the G &D and between the A&E but 15 between the D & A. Their guitar strings were also always the same gouges. Plus they both introduced Phosphor Broke acoustic strings at the same time using the same lists of advantages.

Found an image of some of the strings he had in a gig bag that went to auction.