The base model is a late 1942 to early 1943 Long Branch No4 MkI* (*=wartime expedient bolt release)
The /53 No4 MkI/3 (F) FTR means in 1953 it was converted to the MkII trigger style and received a complete rebuild at the Fazakerely facility.
The MkI when converted is changed to a MkI/2
The MkI* is changed to a MkI/3
These LB 1/3's are relatively rare and $300 would be a fair price. I have seen them go for over $500.
I have a almost identical LB with the 53 1/3 FTR. This LB has had every part except the receiver replaced at FTR, bolt, barrel and all furniture.
Only the Savage and Long Branch No4's can be a MkI/3. These are less than 5% of the No4's on the market.
Someone has unloaded a small number of these 1953 FTR's I have seen more of these in the past 10-12 months than the previous 10 years.
I have a collection of about forty No4's and only one is a LB 1/3, still seeking a Savage 1/3.