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.

Lacquer cuts can usually be identified by "// x ▽420" found stamped into the runout area of vinyl releases. There are occasional exceptions to that rule though: sometimes the triangle or the '420' may be omitted or replaced by letters, or the two forward slashes may be missing.
"// 3 ▽ E420" means the record was cut inhouse.
Being a full service provider, the records were normally also pressed there - for exceptions, see below.

PRS manufactured injection moulded 7"s for the UK market for the Philips and Polydor families of labels. 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 45s (as did Phonodisc Ltd.); these titles require separate entries in the database distinguishing them from the solid centre editions.

Because PRS Ltd. didn't have many lathes, it was common to outsource the lacquer cutting to other studios. Since 1979 those cuts were marked with "▽E" + additional letters, signify the respective mastering studio:
▽E AR = Abbey Road Studios
▽E C = CBS Studios, London
▽E D = Decca Studios
▽E IB = I.B.C. Studios
▽E JVC = JVC Cutting Center
▽E KR = Kendun Recorders
▽E M = Master Room
▽E MC = Music Center, Wembley
▽E MCR = Master Cutting Room
▽E MD = Masterdisk
▽E ML = The Mastering Lab
▽E P = Pye Studios (until 1980)
▽E P = PRT Studios (from 1981)
▽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 UT = Utopia Studios.

From 1982 onwards, the identifiers for external lacquer cuts were changed to "▽420" + 1 additional letter. The letter signifies the respective mastering studio:
▽420 130 = Sterling Sound
▽420 A = Abbey Road Studios
▽420 C = CBS Studios, London
▽420 D = Decca Studios, may also appear as DEC
▽420 E = EMI Studios or (The Penthouse, when explicitly stated)
▽420 EX = The Exchange
▽420 G = Portland Studios
▽420 H = The Town House (may occasionally be credited as M)
▽420 J = JVC Cutting Center
▽420 K = Kendun Recorders
▽420 MU = Masterdisk
▽420 P = PRT Studios
▽420 R = Master Room
▽420 S = Strawberry Mastering
▽420 T = Tape One
▽420 U = Utopia Studios
▽420 V = Trident Studios until 1985
▽420 V = Audio One after 1985
▽420 W = The Music Centre, Wembley
▽420 Z = The Sound Clinic

Other appearances:
▽420 DA = Mastered At and Pressed By Damont
▽420 FRA = Mastered At and Pressed By C.I.D.I.S. Louviers (until April 1979 / PRS, Louviers - often also credited POLYGRAM (since May 1979).
▽420 ORL or ORLAKE = Mastered At and Pressed By Orlake Records
▽420 WEA = Mastered At and Pressed By WEA Records Pressing Plant, West Drayton