It was standard practice during the war to leave gaps in serial numbers to keep jerry guessing about how many we'd produced when he got his hands on some serials, so there're plenty of gaps to choose from, and LM428 was used, on a 50sqn lanc lost to Berlin on 28th January 1944...........
what i'll do beause it's easier is give you all the serial numbers ever issued to lancs in the order of when the production orders were given, just pick a serial number that doesn't fall within any of these ranges, gaps also exist within the serial ranges given, if you want me to try and find an un-issued number within a particular range you like the look/sound/date of i can try that later as well...........
L7276- L7584 Mk.I
R5482- R5763 Mk.I
R5768- R5917 Mk.I (mostly from manchesters)
W4102- W4700 Mk.I
W4761- W5012 Mk.I/III
DS601- DS852 Mk.II
DV155- DV407 Mk.I/III
ED303- EE202 Mk.I/III
FM100- FM299 Mk.X
HK535- HK806 Mk.I
JA672- JB748 Mk.III
KB700- KB999 Mk.X
LL617- LM296 (ordered as Mk.Is and IIIs, delivered as Mk.Is and IIs)
LM301- LM756 Mk.IIIs (including 10 Mk.Is)
ME295- ME551 Mk.I/III
ME554- ME868 Mk.I
ND324- NE181 Mk.III
NF906- NG503 Mk.I
NN694- NN816 Mk.I
NX548- NX974 Mk.I VII
PA158- PA835 Mk.I
PA694- PD196 Mk.I/III (including the B.I Specials)
PD198- PD444 Mk.I
PP663- PP918 Mk.I
RA500- RA806 Mk.I (inlcluding many Lincolns)
RE100- RF119 Mk.I (including many Lincolns)
RF120- RF599 Mk.I/III
RS102- RS225 (never built)
RT140- RT456 (never built)
RT670- RT750 Mk.VII
ST477- ST790 (never built)
SW243- SW279 Mk.I
SW283- SW316 Mk.I/III
SW319- SW377 Mk.III
SX558- SX921 (never built)
TG758- TG799 (never built)
TW647- TW911 Mk.I (FE)
TW915- TW929 Mk.I/I (FE)
TX623- TX290 Mk.III/ Lancastrian C.IV
Numerous Lancastrians and Lincolns followed