PRS Ltd.

PolyGram Record Services Limited (Polygram Record Operations Ltd.), operating between February 1979 and mid 1988.
Manufacturer and distributor with a vinyl pressing plant located in Chingford/Walthamstow, Greater London.
The plant operated under this name from 2 February 1979 (registered 11 May 1979) until mid 1988 and then closed down.
Succeeded Phonodisc Ltd. and was integrated in PolyGram Record Operations on 1 January 1980.

Being a full service provider, records were normally pressed there; for mastering and exceptions see below.

As well as 10" and 12" records, identifiable by the label rings, PRS manufactured injection moulded 7"s for the UK market, not only for the Philips and Polydor families of labels, and also took out contract pressings. Usually, those singles carry Made in England on the labels and have a small spindle hole within a wide black ring.
PRS also produced already dinked 7"s (as did Phonodisc Ltd.) with a large centre hole; these titles require separate entries in the database distinguishing them from the solid-centre editions.

Lacquer cuts can usually be identified by "// x ▽420" and some trailing numbers in relief (from the stampers) found stamped into the runout area of vinyl releases.
There are occasional exceptions to that rule though: sometimes the ▽ or the "420" may be omitted or replaced by letters, or the two forward slashes may be missing.
The letter "E" combined with "420" means the record was cut in-house (e.g. "// 3 ▽ E420").

Because PRS Ltd. only had two lathes, it was common to outsource the lacquer cutting to other studios.
At first external cuts were marked with "▽E" + additional letters, signifying the respective cutting studio:

▽E AR = Abbey Road Studios
▽E C = CBS Studios, London
▽E CRI = Criteria Recording Studios
▽E D = Decca Studios - until 1981
▽E IB = I.B.C. Studios
▽E JVC = JVC Cutting Center
▽E KR = Kendun Recorders
▽E M = Master Room
▽E MC = The Music Centre, Wembley
▽E MCR = Master Cutting Room
▽E MD = Masterdisk
▽E ML = The Mastering Lab
▽E P = Pye Studios - until 1980
▽E P = PRT Studios - 1981 onwards
▽E PS = Portland Studios
▽E R = RCA Studios, London
▽E SC = The Sound Clinic
▽E ST = Sterling Sound
▽E STR = Strawberry Mastering
▽E T = Trident Studios
▽E TH = The Town House
▽E TO = Tape One
▽E USA = Lacquer cut by unspecified studio in the US
▽E UT = Utopia Studios.

The identifiers for external lacquer cuts were still in 1979 changed to "▽420" + 1 additional letter (“▽130” for US studios). The letter signifies the respective cutting studio:
▽420 A = Abbey Road Studios
▽420 AO = Audio One - 1985 onwards
▽420 C = CBS Studios, London
▽420 DEC = Decca Studios - until 1981
▽420 E = EMI Studios or (The Penthouse, when explicitly stated)
▽420 EX = The Exchange
▽420 G = Portland Studios
▽420 H = The Town House
▽420 J = JVC Cutting Center
▽420 K = Kendun Recorders
▽420 L = Dublin Disc Cutting Service
▽420 M = Master Room - 1979 until 1981
▽420 M = CTS Studios - 1982 onwards
▽420 MU = Masterdisk USA
▽420 P = PRT Studios
▽420 R = Master Room - 1982 onwards
▽420 S = Strawberry Mastering - until 1983
▽420 S = CTS Studios - 1984 onwards
▽420 T = Tape One
▽420 U = Utopia Studios
▽420 V = Trident Studios - until 1984
▽420 V = Audio One - 1985 onwards
▽420 W = The Music Centre, Wembley - until 1981
▽420 W = CTS Studios - 1982 onwards
▽420 Z = The Sound Clinic

▽130 MD = lacquers were cut at Masterdisk
▽130 SS = lacquers were cut at Sterling Sound

Contract pressings:

▽420 DA = Lacquer Cut At PRS Ltd. and Pressed By Damont
▽420 FRA or ▽420 FRA POLYGRAM = Lacquer Cut At PRS Ltd. and Pressed By Polygram Industries Messageries, France
▽420 MSL = Lacquer Cut At PRS Ltd. and Pressed By Multiple Sound Pressings Ltd., Leicester
▽420 ORL or ORLAKE = Lacquer Cut At PRS Ltd. and Pressed By Orlake Records
▽420 WEA = Lacquer Cut At PRS Ltd. and Pressed By WEA Records Pressing Plant, West Drayton