#38 - The packing department - Mentioned in S7E5, S7E6 and several others. In S7E5, Mr. Harman says to Captain Peacock, "they're all asking about you in the packing department".

#39 - Catering - In S5E3 "Founders Day", the staff have to come up with a celebration for Young Mr. Grace.

During the canteen lunch, Miss Brahms asks, "Why do we have to think about it?"

Captain Peacock: "It's our turn."

Mrs. Slocombe: "Catering did him last year."

I know that catering was mentioned in other episodes as well.

#40 - Fabrics - These may or may not fall under other departments, however since they are mentioned separately during certain episodes, I'm going to list it as its own department. 

In S6E3, Mr. Tebbs dresses as a Vicor. Before he comes down to the department floor, Mr. Harman says, "He's gone down to fabrics now getting fitted out with some very regal looking curtain material"

#41 - Soft furnishings - Like #40, I'm listing it separately even though it may be part of another department. This was mentioned as part of Mr. Tebbs' background in S6E3:

Mr. Rumbold: "Mr. Tebbs was also 18 months in soft furnishings"

Mr. Tebbs: "Yes, I resigned, when they introduced bean bags."

Lol, I've had bean bag chairs before and always thought they were very comfortable.

#42 - The Patisserie - This is where Mr. Humphries gets the cake for Mrs. Slocombe in S4E5 "Fifty Years On".

#43 - Novelty Candles - Yes, this is actually mentioned as its own department with Miss Hurst being the main staff member. She appears in S8E3 "Front Page Story".

#44 - Gardening furniture - Mentioned in S7E4:

Mr. Humphries: "Oh yes, that's a very strong scent."

Mrs. Comlozi: "I sold that to Mr. Davis, head of gardening furniture"

#45 - Theater bookings - S7E4:

Mrs. Slocombe: "Will I get a key?"

Mr. Harman: "Oh yes, you get the key to the executive kharzi from Mr. Bancroft of theater bookings."

#46 - Flower Store - The place where Captain Peacock has his "standing order" to buy his daily red carnation that he wears in his button hole. Mentioned near the beginning of S5E4.

#47 - Gentlemen's Shoes - This is mentioned as part of Mr. Grainger's background in S2E1. Although the "footwear" department is mentioned a few times throughout the series, it seems that there was a separate department that Mr. Grainger worked in that was much more specialized.

#48 - Carpeting - Mentioned in S6E3 by Mr. Rumbold when telling Mrs. Slocombe about all the places she can get a 'staff discount' in the store.

#49 - Decorating - During S6E2 "The Club", Mr. Harman brings in some excess inventory of wall paper samples for the staff to look at in Mr. Rumbold's office.

Mr. Harman says, "Would the members of the club look through that lot and pick out which ones they would want. It's the stuff they've been trying to get rid of from decorating."

#50 - Cutting - In S8E5 "Heir Apparent", the beginning of the episode introduces Mr. Klein from "cutting", who is temporarily helping out the gents department during their sale.

#51 - Silverware - Captain Peacock says he got his "candle" in silverware in S8E8, which he uses as a flashlight to find his way up to the department floor.

#52 - Security department - This has been mentioned MANY times throughout the series.....for example, in S5E1, Miss Brahms says that no one told security that they were coming in early, which is why they were late for the morning meeting.