Many Philips lamps have item codes marked on the etch, which typically consist of a number followed by a letter, then a slash, and another number.  It appears that these can be used to denote certain construction features which are not otherwise obvious - for instance, what kind of phosphor is used on a mercury lamp.

The letter seems to always indicate the cap type, and the number after the slash indicates whether the bulb is of soft or hard glass, its finish, and the type of coating.  Below is a listing of the types I've come across, which may be helpful for others to identify what they have.  There are obviously some omissions here, so perhaps a more complete list could be built up over time.  The systems are also not perfect - sometimes there are conflicts of the same codes apparently being used for two entirely different finishes, or that one cap type may have used more than one letter over the course of time.

Letters - cap type:
A   European 2-pin plug 19mm
B   Bayonet B22d, BY22d (occasionally also used for B15d)
C   Prefocus G17q, P28s, P30s, P35s, P40s, PG16s, PG20, PG22d, PG28, PG40I, PX22d, PX28s, Pathe (PP10I, PP26, PP27), Debrie (PD27), Precivox (PV27), SAFAR (PS27)
D   Screw E10
E   Screw E27
F   Bayonet B22d-3
G   Screw E40
H   Projector Bell&Howell P38s, P46s
K   Cable
M   Screw E12, E14
N   Bayonet Automotive BA9s, BA15s
P   Glass Projectors G22, G38, GX9.5, GY9.5, P40s, Bernstein 2-pin
R   S15s, R7s
S   G6.35, S39s, GIII8x22t Lighthouse
T   S19, Telephone
U   Bosch BA20d
V   Bosch BA20s
W   Bayonet B15d
X   Special BA24s, BA42, PP20 Pathe, P46s B&H, PS15s, Metal Strip, Specials, British 2A plug
Y   Special BA42, S20s
Z   Special PV27 Precisvox, Specially terminated cables, American flat 2-blade plug

Numbers - Glass Bulb & Coating Type
Code   Glass   Finish
/00   None   None
/00   Any   Clear
/01   Soft   Clear Bamboo
/01   Soft   Clear external silvered
/02   Soft   Clear Bamboo
/02   Soft   Clear Miniature
/04   Any   Clear part silvered
/05   Any   Clear with opaque top
/06   Hard   Clear Mirrored Altrilux
/07   Soft   Clear Miniature
/09   Soft   Fluorescent Woods Blacklight Blue
/10   Soft   Fluorescent Actinic Blue
/11   Quartz   Clear
/13   Soft   External frost & aluminised
/15   Soft   Fluorescent Red
/16   Soft   Fluorescent Yellow
/16   Soft   Clear with decorative white painted neck (neon)
/17   Soft   Fluorescent Green
/18   Soft   Fluorescent Blue
/20   Hard   Clear Biosol
/21   Soft   Internal Frosted
/22   Soft   Clear glass with Sulphide phosphor
/23   -   Clear External side mirror
/24   Soft   Pearl Mirrored Comptalux
/25   Soft   Frosted glass with Magnesium Arsenate phosphor
/28   Hard   Pearl UV-transmitting
/29   Soft   Fluorescent Halophosphate Warm White
/30   Soft   Clear Germicidal TUV
/32   Soft   Fluorescent Halophosphate Warm White Deluxe
/33   Soft   Fluorescent Halophosphate White
/34   Soft   Fluorescent Halophosphate White Deluxe
/37   Soft   Clear Miniature
/43   -   Pearl/Black
/44   Soft   Pearl Mirrored Attralux
/45   Soft   Pearl Miniature
/48   Soft   Fluorescent Neon
/50   Soft   Miniature unknown
/50   Soft   Miniature crown silver
/51   Soft   Opal glass
/51   Quartz   Clear
/54   Soft   Fluorescent Cool Daylight
/55   Soft   Fluorescent Daylight
/56   Soft   Frosted and internal opalised
/6   Hard   Dichroic Refl
/62   Soft   Darkroom Light Red
/63   Soft   Darkroom Dark Red
/63   Any   Fluorescent Magnesium Arsenate Reflector
/69   Soft   Darkroom Light Green
/70   Soft   Woods
/88   Soft   Silica Internal
/89   Soft   Photomirenta
/92   Hard   Tubular.  UV transmitting?
/93   Soft   Pearl with Vanadate phosphor
/94   Hard   Pearl
/95   Hard   UV-transmitting for HOG Mercury
/96   Hard   Pearl and internal opalised
/97   Soft   Blue laquer photoflash
/97   Hard   Clear or Frosted glass with Magnesium Arsenate phosphor
/98   Soft   Clear Mirrored red front Infraphil
/99   Hard   Pearl or Clear mirrored Altrilux / A1
/99   Soft   HPR Mercury
/479   Hard   Ruby Infrared with CdSe vaporised coating